CQ Acids Alkalis
Content
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Acid and alkalis : 1985 3. (b) Three bottles of dilute acid are labelled as X, Y and Z. One of them contains nitric acid, another sulphuric acid and the remaining one hydrochloric acid. The table below summarises the results of two tests carried out on the three acids Observation Test X Y Z Heating with A colourless gas is A colourless Same observation as marble chips rapidly liberated but the gas is rapidly Y. reaction soon slows liberated. down and then stops. Heating with A colourless gas is Same Yellowish-brown fumes zinc granules rapidly liberated, and a observation appear above the colourless solution is as X reaction mixture, and a obtained. colourless solution is obtained. (i) Identify X and Z. In each case, explain the observation that leads to your identification. (ii) (1) Name the colourless solution formed in the reaction between zinc and Y. (2) What will be observed if dilute sodium hydroxide solution is slowly added to the solution in (ii)(1), until the hydroxide is in excess ? (iii) Write an equation for the reaction of dilute nitric acid with (1) marble chips. (2) zinc granules. (9 marks)
Acid and alkalis : 1986 2. (b) (i) Sodium hydrogencarbonate decomposes upon heating to give carbon dioxide. What volume of carbon dioxide at room temperature and atmospheric pressure can be obtained by decomposing 1.00 g of sodium hydrogencarbonate ? (Relative atomic masses : H=1.00, C=12.0, O=16.0, Na = 23.0. Molar volume of a gas at room temperature and atmospheric pressure 24.0 dm3) (ii) Baking powder is a mixture containing sodium hydrogencarbonate and a solid acid, HA. (1) Why should baking powder be stored in a dry place ? (2) Write an equation for the reaction between sodium hydrogencarbonate and the acid HA. (3) Suggest why sodium hydrogencarbonate is used in baking cakes. (iii) Give ONE use of sodium hydrogencarbonate in first aid process (9 marks) 5. (c) 0.27 g of magnesium ribbon was dropped into a conical flask containing 25.0 cm3 of 0.500
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M sulphuric acid. After reaction, the excess acid was titrated against 0.203 M sodium hydroxide solution, and 21.50 cm3 of the base were required for neutralization. (Relative atomic mass of magnesium = 24.3) (i) Write an equation for the reaction between magnesium and sulphuric acid. (ii) Suggest a suitable indicator for this titration, and give the colour change at the endpoint. (iii) From the titration data, calculate (1) the number of moles of sodium hydroxide used to neutralize the excess acid. (2) the number of moles of sulphuric acid which have reacted with the magnesium. (3) the percentage purity of the magnesium ribbon. (iv) According to the manufacturer, the purity of the magnesium ribbon was 99.6%, but the purity determined in (iii) (3) was lower than this value. (1) Suggest a possible reason for this difference. (2) Suggest how the experimental procedure could be improved to obtain a purity determination closer to 99.6%. (10 marks)
Acid and alkalis : 1988 1. (b) The chart below shows two routes by which atmospheric nitrogen is converted to fertilizers that can be used by plants: (i) What is the natural process that can convert atmospheric nitrogen to nitric acid in Route I ? Write balanced equations to illustrate this process. (ii) (1) Name the industrial process for the conversion of atmospheric nitrogen to ammonia in Route II, and write a balanced equation for the conversion. (2) Knowing that this process is exothermic, give TWO conditions that can theoretically give a high yield of ammonia and explain your answer. (iii) Assuming that the effectiveness of a fertilizer is directly proportional to its nitrogen content per unit mass, determine by calculation whether ammonium nitrate [NH4NO3] or urea [(NH2)2 CO] is the more effective fertilizer. (Relative atomic masses : H=1, C=12, N=14, O=16) (13 marks)
Acid and alkalis : 1989 2. (a)
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Antacid tablets are used to relieve pains caused by excess acid in the stomach. The label (in English and Chinese) on a brand of antacid, called “Consonant”, is shown in the diagram below: CONSONANT® Antacid Tablets Active ingredient : Calcium carbonate Dosage : 1-2 tablets, 3 times a day Chew before swallowing The following experiment was performed by a student to find out the amount of calcium carbonate in such a tablet : Steps (1) A tablet was weighed and ground to a powder, and then added to a conical flask containing some distilled water. (2) A few drops of methyl orange indicator were added to the flask. Standard hydrochloric acid was then added from a burette in small portions, with swirling. (3) On adding each portion of the acid, the solution in the flask first turned pink and then yellow upon swirling. Effervescence was also observed. (4) More acid was added until a permanent pink colour was just observed. Experimental Results: Mass of one tablet = 1.20 g Volume of the hydrochloric acid used = 15.2 cm3 (i) Explain the meaning of the term “standard” in step (2). (ii) In step (3), explain (1) the colour changes in the solution. (2) why effervescence was observed. (iii) In step (4), explain why a permanent pink colour was observed. (iv)Calculate the percentage by mass of calcium carbonate in one antacid tablet. (Relative atomic masses : C = 12.0, O = 16.0, Ca = 4.0.0) (v) On the label, it was recommended that the tablets should be chewed before swallowing. Suggest an explanation for this. (vi)Antacid tablets containing calcium carbonate are seldom used nowadays. Instead, magnesium hydroxide is usually used as the active ingredient. Based on the reactions involved, suggest one reason why magnesium hydroxide is preferred to calcium carbonate. (13 marks) Acid and alkalis : 1990 3. (b) The formula of a weak alkanoic acid can be represented by CnH2nCO2H (where n is an integer).
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A sample of the alkanoic acid weighing 0.355 g was dissolved in about 20 cm3 of water in a conical flask. The solution was then titrated against a 0.180 M sodium hydroxide solution. A total of 22.40 cm3 of the alkali was required for complete neutralization. (i) Explain the meaning of the term ‘weak acid’. (ii) Describe how the end-point in this titration can be determined. (iii) Calculate (1) the number of moles of sodium hydroxide used for the titration. (2) the relative molecular mass of the alkanoic acid. (iv) (1) Deduce the molecular formula of the alkanoic acid. (2) Draw TWO molecular structures for the alkanoic acid. [Relative atomic masses : H=1.0, C=12.0, O=16.0] (12 marks) Acid and alkalis : 1991 2. (a) A student wished to find out which of the two commercial brands of vinegar, A and B, was the better buy, i.e. of lower price per gram of ethanoic acid (CH3COOH). The following table listed some of the information about these two brands: Brand A B Price Volume of vinegar $3.00 250 cm3 $6.00 500 cm3 Concentration of ethanoic acid 50 g dm3 UNKNOWN
The student carried out a titration experiment to determine the concentration of ethanoic acid in Brand B as follows: 25.0 cm3 of the vinegar was first diluted to 250.0 cm3 with distilled water. 25.0 cm3 portions of the diluted solution were then titrated against 0.10 M sodium hydroxide solution, using a suitable indicator, until the end-point was reached. The following results were obtained. Titration Final reading (cm3) Initial reading (cm3) 1 2 5.5 0 0 .00 2 2 5.7 0 1 .00 3 2 6.2 0 1 .30 4 2 5.9 0 1 .10
(i) Describe, giving the names of the apparatus used, how 25.0 cm3 of the vinegar should be diluted to 250.0 cm3. (ii) Suggest a suitable indicator for this titration and state its colour change at the endpoint. (iii) Based on the titration results, calculate a reasonable average for the volume of the sodium hydroxide solution used. (iv) Write the equation for this titration. (Ionic equations will not be accepted.) (v) Calculate the molarity of ethanoic acid in Brand B. (vi) Show by calculation which brand of vinegar is the better buy. (Relative atomic masses: H=1.0; C=12.0; O=16.0) (13 marks)
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Acid and alkalis : 1992 1. (a) A student tried to prepare a sample of a solid salt by reacting copper(II) oxide with sulphuric acid in the laboratory as shown in the diagram below. The student wrote the following procedure of the experiment in her notebook: I. II. III. IV. Excess copper(Il) oxide was added to 50.0 cm3 of 2.0 M sulphuric acid in a beaker. The mixture was heated for 2 minutes, and was stirred continuously during this time. The remaining copper(II) oxide was filtered off. The filtrate was allowed to cool for one day.
(i) Referring to the above diagram, write down TWO aspects that are considered UNSAFE in the laboratory. (ii) (1) Name the salt the student tried to prepare. (2) Calculate the theoretical mass of the salt that can be obtained. (iii)Explain why the student heated the reaction mixture in step II. (iv) The student followed exactly the procedure written in her notebook, but did not obtain any SOLID salt after one day. Suggest an explanation. (Relative atomic masses : H=1.0; O=16.0; S=32.0; Cu=63.5) (8 marks) 2. (a) Consider the following hazard warning labels :
Label A
Label B
Label C
Label D
Choose TWO labels which should be displayed on a bottle of concentrated nitric acid. Explain your choice. (3 marks)
2. (b) What would be observed when concentrated nitric acid is added dropwise to (i) a saturated solution of iron(II) sulphate? (ii) zinc granules? Explain the observations and write appropriate equations. (6 marks) 2. (c)
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1.0 g of calcium carbonate is added to 50.0 cm3 of 0.1 M nitric acid. At the end of the reaction, 55.0 cm3 of a certain gas are collected at room temperature and pressure. (i) Draw a diagram of the set-up suitable for this experiment. (ii) Calculate the theoretical volume of the gas which would be liberated at room temperature and pressure. (iii)Explain any difference between the theoretical volume and the volume of the gas collected. (Relative atomic masses : C=12.0; O= 16.0; Ca=40.0; Molar volume of gas at room temperature and pressure = 24.0 dm3) (6 marks) Acid and alkalis : 1993 1. (b) Liquid wastes discharged from some factories are acidic and need to be neutralized before discharging into the sea. A certain factory used slaked lime (calcium hydroxide) to neutralize its liquid waste, which consisted of 0.5 M hydrochloric acid discharging at a rate of 20 dm3 per minute. (i) Why are the liquid wastes neutralized before discharging into the sea ? (ii) Write an equation for the reaction between hydrochloric acid and slaked lime. (iii) Calculate the mass of slaked lime required per minute to neutralize the acid present in the liquid wastes. (iv) Although slaked lime is cheaper, factories nowadays use sodium carbonate instead of slaked lime to neutralize their acidic liquid wastes. Suggest a reason. (Relative atomic masses : H=1.0; O=16.0; Ca=40.0) (6 marks) 4. (b) To determine the percentage by mass of calcium carbonate in egg shells, a student added 100 cm3 of 2 M hydrochloric acid to 0.3 g of egg shells in a container. After 30 minutes, all the egg shells dissolved and 67 cm3 of carbon dioxide were collected at room temperature and pressure. (i) Write an equation for the reaction between calcium carbonate and hydrochloric acid. (ii) Calculate the percentage by mass of calcium carbonate in the egg shells. (Relative atomic masses : H=1.0; C=12.0; O=16.0; Ca=40.0; Molar volume of gas at room temperature and pressure = 24.0 dm3) (4 marks) Acid and alkalis : 1994 5. (a) A domestic drain cleaner named ‘RAINBOW’ contains concentrated sulphuric acid as the active ingredient. A student carried out the following experiment to determine the concentration of sulphuric acid in ‘RAINBOW’. 1.0 cm3 of ‘RAINBOW’ was diluted to 500 cm3 with distilled water. 25.0 cm3 of the diluted solution were measured and transferred to a conical flask. The solution in the flask required 18.2 cm3 of 0.10 M sodium hydroxide solution for complete
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neutralization. (i) Name the apparatus used to measure 25.0 cm3 of the diluted solution. (ii) Calculate the molarity of sulphuric acid in ‘RAINBOW’. (iii)Suggest ONE disadvantage of using ‘RAINBOW’ for cleaning drains. (iv) State ONE safety precaution needed when using ‘RAINBOW’. Explain your answer. (v) If ‘RAINBOW’ is poured into drains blocked with fat, the fat can be removed. Assuming the formula of fat is H | H—C—OCOR | H—C—OCOR (R represents an alkyl group), | H—C—OCOR | H explain how ‘RAINBOW’ can remove the fat. (8 marks) Acid and alkalis : 1995 7. (a) The label on a bottle of ‘Effervescent Calcium’ tablets is shown below. (i) Effervescence occurs when a tablet of ‘Effervescent Calcium’ is added to water. Based on the information given on the label, explain why effervescence occurs. Write the ionic equation for the reaction that occurs. (ii) Suppose that a student puts a tablet of ‘Effervescent Calcium’ into an excess amount of water and collects the gas liberated. Effervescent Calcium Each bottle contains 10 tablets. Each tablet contains Calcium carbonate 625 mg Vitamin C 1000 mg Citric acid 1350 mg Dosage 1 tablet daily Administration : Dissolve one tablet in a glass of water. Warning : (1) Keep out of reach of children. (2) Keep …………………..
(1) Assuming that the tablet completely dissolves, calculate the theoretical volume of gas liberated. (2) It is found that the volume of gas collected in the experiment is less than the theoretical volume calculated in (1). Give ONE reason to explain the difference, assuming that there is no leakage of gas in the experiment. (iii)On the label, some words are missing in the second warning statement. Complete the second warning statement, beginning with the word ‘Keep’. Explain your answer. (Relative atomic masses C=12.0, O=16.0, Ca=40.0; Molar volume of gas at room temperature and pressure = 24.0 dm3) (9 marks) Acid and alkalis : 1996 6. (b)
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A , B, C and D are four unlabelled bottles, each containing one of the following reagents: 2M ammonia solution, 2M ethanoic acid, 2M hydrochloric acid, 2M nitric acid The following scheme is uscd to identify the four reagents. (i) What is the reagent in bottle A ? Explain why this reagent turns red litmus paper blue. (ii) What is the reagent in bottle B ? Write a chemical equation for the reaction between this reagent and copper turnings, and a chemical equation for the formation of the brown fumes. (iii) (1) Suggest a test to distinguish between the reagents in bottles C and D. (Smelling the reagents is NOT an acceptable answer.) (2) State the observable change in this test and explain your answer. (8 marks) 9. (a) The diagram below shows a longitudinal section of a fire extinguisher. When the extinguisher is being used, solution X is allowed to mix with the sodium hydrogencarbonate solution and a reaction immediately occurs. A jet of aqueous solution will then be forced out of the extinguisher. (i) Suggest what solution X may be. (ii) (1) Write an ionic equation for the reaction between solution X and the sodium hydrogencarbonate solution. (2) Explain why a jet of aqueous solution is forced out of the extinguisher. (3 marks) Acid and alkalis : 1997 3. (a) Suggest ONE method to determine the pH of an aqueous solution. (b) Arrange the following substances in the order of increasing pH and explain your answer. 1M ethanoic acid, 1M hydrochloric acid, 1M sulphuric acid (c) Three solution samples, A, B and C, are being tested for the purpose of perming hair. The table below lists the pH of these samples : Sample pH A 6.5 B 8.5
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C 10.0 Which sample is most suitable for perming hair answer ? Explain your (6 marks) 7. (a) Malachite is a mineral containing copper(Il) carbonate and copper(II) hydroxide. It is insoluble in water but reacts with dilute sulphuric acid. The procedure for preparing copper(II) sulphate crystals from malachite is as follows. Step 1 Step 2 Step 3 Step 4 Pour 50 cm3 of 2M sulphuric acid in a beaker and then warm the acid. Add small portions of powdered malachite to the warm acid while constantly stirring, until effervescence stops and some powdered malachite remains in the beaker. Remove the remaining powdered malachite from the solution. Evaporate the solution slowly to obtain copper(II) sulphate crystals.
(i) Write a chemical equation for the reaction which causes the effervescence. (ii) Why is it necessary to add powdered malachite until some of it remains in the beaker? (iii)Draw a labelled diagram to show how the remaining powdered malachite can be removed from the solution. (iv) Calculate the theoretical mass of copper(II) sulphate crystals, CuSO4.5H20, that can be obtaincd. (Relative atomic masses : H=1.0, C=12.0, O=16.0, S=32.1, Cu=63.5) (8 mark) Acid and alkalis : 1998 3. Consider the following substances: Sodium benzoate, sodij chloride, sodium pypochlorite, Sodium hydrogencarbonate, sodium hydroxide, Sodium sulphite and monsodium glutamate (c) Which substance is commonly used as an active ingredient in oven cleaners? Briefly explain its action. (3 marks) 5. Each of five unlabelled bottles contains one of the following chemicals: 2M hydrochloric acid 2M sodium chloride solution distilled water 2M nitric acid 2M sodium hydroxide solution
Suggest how you would carry out tests to identify the contents of each bottle, using the materials and apparatus listed below. Your answer should include the observation of each test. copper foil, solid copper(II) carbonate, 2M copper(II) chloride solution, test tubes and a Bunsen burner (You are NOT required to write chemical equations. Answers in the form of flow diagrams will NOT be marked.)
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(8 marks) 6. (a) (i) A student prepared sodium nitrate solution by reacting 1M sodium hydroxide solution with dilute nitric acid. The student carried out a titration to determine the amount of dilute nitric acid required to react with a known volume of 1M sodium hydroxide solution. (1) Write the chemical equation for the reaction. (An ionic equation will NOT be accepted for this question.) (2) Draw a labelled diagram for the set-up of the titration. (3) Phenolphthalein can be used to determine the end point of the titration. State the colour change at the end point. (4) Suggest how the student can prepare a sodium nitrate solution using the titration results. (5 marks)
Acid and alkalis : 1999 2. For each of the following experiments, state ONE observable change and write a chemical equation for the reaction involved. (a) Dilute nitric acid is added to magnesium carbonate powder in a beaker. (2 marks) 5. The diagram below shows a bottle of chemical waste in a school laboratory. Describe and explain how you would remove kerosene and iron(III) ions from the chemical waste. (You may use any apparatus and chemicals available in a school laboratory.)
k e ro se n e a n a q u e o u s s o lu tio n o f iro n (III) c h lo rid e a n d s o d iu m c h lo rid e
(8 marks) 7. (b) A student carried out the following experiment to determine the concentration of ethanoic acid in a brand of commercial vinegar. ’25.0 cm3 of the vinegar was placed in apparatus Y and was then diluted to 25.0 cm3 with distilled water. 25.0 cm3 of the diluted vinegar solution was withdrawn and titrated against 0.10 M sodium hydroxide solution with phenolphthalein as indicator.’ (i) Write a half-equation for the reaction that occurs at the zinc case of the cell during discharge. (ii) State the function of following substances in a zinc-carbon cell. (1) carbon rod (2) manganese(IV) oxide (iii) Suggest a chemical test to show the presence of ammonium ions in the moist paste of ammonium chloride. (iv) Explain whether you agree with the following statement.
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‘Zinc-carbon cells cause more environmental problems than nickel-cadmium cells do.’ (v) Complete and balance the following half-equations for the reactions that occur at the electrodes of a nickel-cadmium cell. Cd + OH- → Cd(OH)2 NiO2 + H2O → Ni(OH)2 + OH(9 marks) Acid and alkalis : 2000 7. (a) The sour taste of citrus fruits is mainly due to citric acid which is a weak acid with the following structure:
CH2CO2H HO C CO2H
(i) What is the meaning of the term ‘weak acid’? CH2CO2H (ii) Given that citric acid is a tribasic acid, write a chemical equation for the complete neutralization of nitric acid by sodium hydroxide solution. (iii) In a titration experiment to determine the concentration of citric acid in orange juice, 25.0 cm3 of a sample of orange juice required 10.0 cm3 of 0.30 M sodium hydroxide solution to reach the end-point, at which the citric acid had been completely neutralized. (1) Given that the relative molecular mass of citric acid is 192, calculate the concentration of citric acid, in g dm-3, in the sample of orange juice. (2) State ONE assumption in your calculation in (1). (3) Given that the following materials and apparatus, briefly describe how a 0.30 M sodium hydroxide solution can be prepared. 1.20 M sodium hydroxide solution distilled water 25.0 cm3 pipette 50.0 cm3 volumetric flask, 100.0 cm3 volumetric flask and 250.0 cm3 volumetric flask (Only one of these flasks should be used.) (10 marks) 4. The mass of a sample of copper powder contaminated with copper(II) oxide is known. Describe how you would conduct an experiment to derermine the percentage by mass of copper powder in the sample. State the expected observation of the experiment. (Hint: You may use an acid in the experiment.) (9 marks) 8. (c) State whether each of the following statements is true or false. Explain your answer in each case. (ii) When concentrated sulphuric acid is diluted, water should be added slowly to the acid. (2 marks)
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Acid and alkalis : 1985 3. (b) Three bottles of dilute acid are labelled as X, Y and Z. One of them contains nitric acid, another sulphuric acid and the remaining one hydrochloric acid. The table below summarises the results of two tests carried out on the three acids Observation Test X Y Z Heating with A colourless gas is A colourless Same observation as marble chips rapidly liberated but the gas is rapidly Y. reaction soon slows liberated. down and then stops. Heating with A colourless gas is Same Yellowish-brown fumes zinc granules rapidly liberated, and a observation appear above the colourless solution is as X reaction mixture, and a obtained. colourless solution is obtained. (i) Identify X and Z. In each case, explain the observation that leads to your identification. (ii) (1) Name the colourless solution formed in the reaction between zinc and Y. (2) What will be observed if dilute sodium hydroxide solution is slowly added to the solution in (ii)(1), until the hydroxide is in excess ? (iii) Write an equation for the reaction of dilute nitric acid with (1) marble chips. (2) zinc granules. (9 marks)
Acid and alkalis : 1986 2. (b) (i) Sodium hydrogencarbonate decomposes upon heating to give carbon dioxide. What volume of carbon dioxide at room temperature and atmospheric pressure can be obtained by decomposing 1.00 g of sodium hydrogencarbonate ? (Relative atomic masses : H=1.00, C=12.0, O=16.0, Na = 23.0. Molar volume of a gas at room temperature and atmospheric pressure 24.0 dm3) (ii) Baking powder is a mixture containing sodium hydrogencarbonate and a solid acid, HA. (1) Why should baking powder be stored in a dry place ? (2) Write an equation for the reaction between sodium hydrogencarbonate and the acid HA. (3) Suggest why sodium hydrogencarbonate is used in baking cakes. (iii) Give ONE use of sodium hydrogencarbonate in first aid process (9 marks) 5. (c) 0.27 g of magnesium ribbon was dropped into a conical flask containing 25.0 cm3 of 0.500
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M sulphuric acid. After reaction, the excess acid was titrated against 0.203 M sodium hydroxide solution, and 21.50 cm3 of the base were required for neutralization. (Relative atomic mass of magnesium = 24.3) (i) Write an equation for the reaction between magnesium and sulphuric acid. (ii) Suggest a suitable indicator for this titration, and give the colour change at the endpoint. (iii) From the titration data, calculate (1) the number of moles of sodium hydroxide used to neutralize the excess acid. (2) the number of moles of sulphuric acid which have reacted with the magnesium. (3) the percentage purity of the magnesium ribbon. (iv) According to the manufacturer, the purity of the magnesium ribbon was 99.6%, but the purity determined in (iii) (3) was lower than this value. (1) Suggest a possible reason for this difference. (2) Suggest how the experimental procedure could be improved to obtain a purity determination closer to 99.6%. (10 marks)
Acid and alkalis : 1988 1. (b) The chart below shows two routes by which atmospheric nitrogen is converted to fertilizers that can be used by plants: (i) What is the natural process that can convert atmospheric nitrogen to nitric acid in Route I ? Write balanced equations to illustrate this process. (ii) (1) Name the industrial process for the conversion of atmospheric nitrogen to ammonia in Route II, and write a balanced equation for the conversion. (2) Knowing that this process is exothermic, give TWO conditions that can theoretically give a high yield of ammonia and explain your answer. (iii) Assuming that the effectiveness of a fertilizer is directly proportional to its nitrogen content per unit mass, determine by calculation whether ammonium nitrate [NH4NO3] or urea [(NH2)2 CO] is the more effective fertilizer. (Relative atomic masses : H=1, C=12, N=14, O=16) (13 marks)
Acid and alkalis : 1989 2. (a)
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Antacid tablets are used to relieve pains caused by excess acid in the stomach. The label (in English and Chinese) on a brand of antacid, called “Consonant”, is shown in the diagram below: CONSONANT® Antacid Tablets Active ingredient : Calcium carbonate Dosage : 1-2 tablets, 3 times a day Chew before swallowing The following experiment was performed by a student to find out the amount of calcium carbonate in such a tablet : Steps (1) A tablet was weighed and ground to a powder, and then added to a conical flask containing some distilled water. (2) A few drops of methyl orange indicator were added to the flask. Standard hydrochloric acid was then added from a burette in small portions, with swirling. (3) On adding each portion of the acid, the solution in the flask first turned pink and then yellow upon swirling. Effervescence was also observed. (4) More acid was added until a permanent pink colour was just observed. Experimental Results: Mass of one tablet = 1.20 g Volume of the hydrochloric acid used = 15.2 cm3 (i) Explain the meaning of the term “standard” in step (2). (ii) In step (3), explain (1) the colour changes in the solution. (2) why effervescence was observed. (iii) In step (4), explain why a permanent pink colour was observed. (iv)Calculate the percentage by mass of calcium carbonate in one antacid tablet. (Relative atomic masses : C = 12.0, O = 16.0, Ca = 4.0.0) (v) On the label, it was recommended that the tablets should be chewed before swallowing. Suggest an explanation for this. (vi)Antacid tablets containing calcium carbonate are seldom used nowadays. Instead, magnesium hydroxide is usually used as the active ingredient. Based on the reactions involved, suggest one reason why magnesium hydroxide is preferred to calcium carbonate. (13 marks) Acid and alkalis : 1990 3. (b) The formula of a weak alkanoic acid can be represented by CnH2nCO2H (where n is an integer).
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A sample of the alkanoic acid weighing 0.355 g was dissolved in about 20 cm3 of water in a conical flask. The solution was then titrated against a 0.180 M sodium hydroxide solution. A total of 22.40 cm3 of the alkali was required for complete neutralization. (i) Explain the meaning of the term ‘weak acid’. (ii) Describe how the end-point in this titration can be determined. (iii) Calculate (1) the number of moles of sodium hydroxide used for the titration. (2) the relative molecular mass of the alkanoic acid. (iv) (1) Deduce the molecular formula of the alkanoic acid. (2) Draw TWO molecular structures for the alkanoic acid. [Relative atomic masses : H=1.0, C=12.0, O=16.0] (12 marks) Acid and alkalis : 1991 2. (a) A student wished to find out which of the two commercial brands of vinegar, A and B, was the better buy, i.e. of lower price per gram of ethanoic acid (CH3COOH). The following table listed some of the information about these two brands: Brand A B Price Volume of vinegar $3.00 250 cm3 $6.00 500 cm3 Concentration of ethanoic acid 50 g dm3 UNKNOWN
The student carried out a titration experiment to determine the concentration of ethanoic acid in Brand B as follows: 25.0 cm3 of the vinegar was first diluted to 250.0 cm3 with distilled water. 25.0 cm3 portions of the diluted solution were then titrated against 0.10 M sodium hydroxide solution, using a suitable indicator, until the end-point was reached. The following results were obtained. Titration Final reading (cm3) Initial reading (cm3) 1 2 5.5 0 0 .00 2 2 5.7 0 1 .00 3 2 6.2 0 1 .30 4 2 5.9 0 1 .10
(i) Describe, giving the names of the apparatus used, how 25.0 cm3 of the vinegar should be diluted to 250.0 cm3. (ii) Suggest a suitable indicator for this titration and state its colour change at the endpoint. (iii) Based on the titration results, calculate a reasonable average for the volume of the sodium hydroxide solution used. (iv) Write the equation for this titration. (Ionic equations will not be accepted.) (v) Calculate the molarity of ethanoic acid in Brand B. (vi) Show by calculation which brand of vinegar is the better buy. (Relative atomic masses: H=1.0; C=12.0; O=16.0) (13 marks)
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Acid and alkalis : 1992 1. (a) A student tried to prepare a sample of a solid salt by reacting copper(II) oxide with sulphuric acid in the laboratory as shown in the diagram below. The student wrote the following procedure of the experiment in her notebook: I. II. III. IV. Excess copper(Il) oxide was added to 50.0 cm3 of 2.0 M sulphuric acid in a beaker. The mixture was heated for 2 minutes, and was stirred continuously during this time. The remaining copper(II) oxide was filtered off. The filtrate was allowed to cool for one day.
(i) Referring to the above diagram, write down TWO aspects that are considered UNSAFE in the laboratory. (ii) (1) Name the salt the student tried to prepare. (2) Calculate the theoretical mass of the salt that can be obtained. (iii)Explain why the student heated the reaction mixture in step II. (iv) The student followed exactly the procedure written in her notebook, but did not obtain any SOLID salt after one day. Suggest an explanation. (Relative atomic masses : H=1.0; O=16.0; S=32.0; Cu=63.5) (8 marks) 2. (a) Consider the following hazard warning labels :
Label A
Label B
Label C
Label D
Choose TWO labels which should be displayed on a bottle of concentrated nitric acid. Explain your choice. (3 marks)
2. (b) What would be observed when concentrated nitric acid is added dropwise to (i) a saturated solution of iron(II) sulphate? (ii) zinc granules? Explain the observations and write appropriate equations. (6 marks) 2. (c)
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1.0 g of calcium carbonate is added to 50.0 cm3 of 0.1 M nitric acid. At the end of the reaction, 55.0 cm3 of a certain gas are collected at room temperature and pressure. (i) Draw a diagram of the set-up suitable for this experiment. (ii) Calculate the theoretical volume of the gas which would be liberated at room temperature and pressure. (iii)Explain any difference between the theoretical volume and the volume of the gas collected. (Relative atomic masses : C=12.0; O= 16.0; Ca=40.0; Molar volume of gas at room temperature and pressure = 24.0 dm3) (6 marks) Acid and alkalis : 1993 1. (b) Liquid wastes discharged from some factories are acidic and need to be neutralized before discharging into the sea. A certain factory used slaked lime (calcium hydroxide) to neutralize its liquid waste, which consisted of 0.5 M hydrochloric acid discharging at a rate of 20 dm3 per minute. (i) Why are the liquid wastes neutralized before discharging into the sea ? (ii) Write an equation for the reaction between hydrochloric acid and slaked lime. (iii) Calculate the mass of slaked lime required per minute to neutralize the acid present in the liquid wastes. (iv) Although slaked lime is cheaper, factories nowadays use sodium carbonate instead of slaked lime to neutralize their acidic liquid wastes. Suggest a reason. (Relative atomic masses : H=1.0; O=16.0; Ca=40.0) (6 marks) 4. (b) To determine the percentage by mass of calcium carbonate in egg shells, a student added 100 cm3 of 2 M hydrochloric acid to 0.3 g of egg shells in a container. After 30 minutes, all the egg shells dissolved and 67 cm3 of carbon dioxide were collected at room temperature and pressure. (i) Write an equation for the reaction between calcium carbonate and hydrochloric acid. (ii) Calculate the percentage by mass of calcium carbonate in the egg shells. (Relative atomic masses : H=1.0; C=12.0; O=16.0; Ca=40.0; Molar volume of gas at room temperature and pressure = 24.0 dm3) (4 marks) Acid and alkalis : 1994 5. (a) A domestic drain cleaner named ‘RAINBOW’ contains concentrated sulphuric acid as the active ingredient. A student carried out the following experiment to determine the concentration of sulphuric acid in ‘RAINBOW’. 1.0 cm3 of ‘RAINBOW’ was diluted to 500 cm3 with distilled water. 25.0 cm3 of the diluted solution were measured and transferred to a conical flask. The solution in the flask required 18.2 cm3 of 0.10 M sodium hydroxide solution for complete
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neutralization. (i) Name the apparatus used to measure 25.0 cm3 of the diluted solution. (ii) Calculate the molarity of sulphuric acid in ‘RAINBOW’. (iii)Suggest ONE disadvantage of using ‘RAINBOW’ for cleaning drains. (iv) State ONE safety precaution needed when using ‘RAINBOW’. Explain your answer. (v) If ‘RAINBOW’ is poured into drains blocked with fat, the fat can be removed. Assuming the formula of fat is H | H—C—OCOR | H—C—OCOR (R represents an alkyl group), | H—C—OCOR | H explain how ‘RAINBOW’ can remove the fat. (8 marks) Acid and alkalis : 1995 7. (a) The label on a bottle of ‘Effervescent Calcium’ tablets is shown below. (i) Effervescence occurs when a tablet of ‘Effervescent Calcium’ is added to water. Based on the information given on the label, explain why effervescence occurs. Write the ionic equation for the reaction that occurs. (ii) Suppose that a student puts a tablet of ‘Effervescent Calcium’ into an excess amount of water and collects the gas liberated. Effervescent Calcium Each bottle contains 10 tablets. Each tablet contains Calcium carbonate 625 mg Vitamin C 1000 mg Citric acid 1350 mg Dosage 1 tablet daily Administration : Dissolve one tablet in a glass of water. Warning : (1) Keep out of reach of children. (2) Keep …………………..
(1) Assuming that the tablet completely dissolves, calculate the theoretical volume of gas liberated. (2) It is found that the volume of gas collected in the experiment is less than the theoretical volume calculated in (1). Give ONE reason to explain the difference, assuming that there is no leakage of gas in the experiment. (iii)On the label, some words are missing in the second warning statement. Complete the second warning statement, beginning with the word ‘Keep’. Explain your answer. (Relative atomic masses C=12.0, O=16.0, Ca=40.0; Molar volume of gas at room temperature and pressure = 24.0 dm3) (9 marks) Acid and alkalis : 1996 6. (b)
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A , B, C and D are four unlabelled bottles, each containing one of the following reagents: 2M ammonia solution, 2M ethanoic acid, 2M hydrochloric acid, 2M nitric acid The following scheme is uscd to identify the four reagents. (i) What is the reagent in bottle A ? Explain why this reagent turns red litmus paper blue. (ii) What is the reagent in bottle B ? Write a chemical equation for the reaction between this reagent and copper turnings, and a chemical equation for the formation of the brown fumes. (iii) (1) Suggest a test to distinguish between the reagents in bottles C and D. (Smelling the reagents is NOT an acceptable answer.) (2) State the observable change in this test and explain your answer. (8 marks) 9. (a) The diagram below shows a longitudinal section of a fire extinguisher. When the extinguisher is being used, solution X is allowed to mix with the sodium hydrogencarbonate solution and a reaction immediately occurs. A jet of aqueous solution will then be forced out of the extinguisher. (i) Suggest what solution X may be. (ii) (1) Write an ionic equation for the reaction between solution X and the sodium hydrogencarbonate solution. (2) Explain why a jet of aqueous solution is forced out of the extinguisher. (3 marks) Acid and alkalis : 1997 3. (a) Suggest ONE method to determine the pH of an aqueous solution. (b) Arrange the following substances in the order of increasing pH and explain your answer. 1M ethanoic acid, 1M hydrochloric acid, 1M sulphuric acid (c) Three solution samples, A, B and C, are being tested for the purpose of perming hair. The table below lists the pH of these samples : Sample pH A 6.5 B 8.5
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C 10.0 Which sample is most suitable for perming hair answer ? Explain your (6 marks) 7. (a) Malachite is a mineral containing copper(Il) carbonate and copper(II) hydroxide. It is insoluble in water but reacts with dilute sulphuric acid. The procedure for preparing copper(II) sulphate crystals from malachite is as follows. Step 1 Step 2 Step 3 Step 4 Pour 50 cm3 of 2M sulphuric acid in a beaker and then warm the acid. Add small portions of powdered malachite to the warm acid while constantly stirring, until effervescence stops and some powdered malachite remains in the beaker. Remove the remaining powdered malachite from the solution. Evaporate the solution slowly to obtain copper(II) sulphate crystals.
(i) Write a chemical equation for the reaction which causes the effervescence. (ii) Why is it necessary to add powdered malachite until some of it remains in the beaker? (iii)Draw a labelled diagram to show how the remaining powdered malachite can be removed from the solution. (iv) Calculate the theoretical mass of copper(II) sulphate crystals, CuSO4.5H20, that can be obtaincd. (Relative atomic masses : H=1.0, C=12.0, O=16.0, S=32.1, Cu=63.5) (8 mark) Acid and alkalis : 1998 3. Consider the following substances: Sodium benzoate, sodij chloride, sodium pypochlorite, Sodium hydrogencarbonate, sodium hydroxide, Sodium sulphite and monsodium glutamate (c) Which substance is commonly used as an active ingredient in oven cleaners? Briefly explain its action. (3 marks) 5. Each of five unlabelled bottles contains one of the following chemicals: 2M hydrochloric acid 2M sodium chloride solution distilled water 2M nitric acid 2M sodium hydroxide solution
Suggest how you would carry out tests to identify the contents of each bottle, using the materials and apparatus listed below. Your answer should include the observation of each test. copper foil, solid copper(II) carbonate, 2M copper(II) chloride solution, test tubes and a Bunsen burner (You are NOT required to write chemical equations. Answers in the form of flow diagrams will NOT be marked.)
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(8 marks) 6. (a) (i) A student prepared sodium nitrate solution by reacting 1M sodium hydroxide solution with dilute nitric acid. The student carried out a titration to determine the amount of dilute nitric acid required to react with a known volume of 1M sodium hydroxide solution. (1) Write the chemical equation for the reaction. (An ionic equation will NOT be accepted for this question.) (2) Draw a labelled diagram for the set-up of the titration. (3) Phenolphthalein can be used to determine the end point of the titration. State the colour change at the end point. (4) Suggest how the student can prepare a sodium nitrate solution using the titration results. (5 marks)
Acid and alkalis : 1999 2. For each of the following experiments, state ONE observable change and write a chemical equation for the reaction involved. (a) Dilute nitric acid is added to magnesium carbonate powder in a beaker. (2 marks) 5. The diagram below shows a bottle of chemical waste in a school laboratory. Describe and explain how you would remove kerosene and iron(III) ions from the chemical waste. (You may use any apparatus and chemicals available in a school laboratory.)
k e ro se n e a n a q u e o u s s o lu tio n o f iro n (III) c h lo rid e a n d s o d iu m c h lo rid e
(8 marks) 7. (b) A student carried out the following experiment to determine the concentration of ethanoic acid in a brand of commercial vinegar. ’25.0 cm3 of the vinegar was placed in apparatus Y and was then diluted to 25.0 cm3 with distilled water. 25.0 cm3 of the diluted vinegar solution was withdrawn and titrated against 0.10 M sodium hydroxide solution with phenolphthalein as indicator.’ (i) Write a half-equation for the reaction that occurs at the zinc case of the cell during discharge. (ii) State the function of following substances in a zinc-carbon cell. (1) carbon rod (2) manganese(IV) oxide (iii) Suggest a chemical test to show the presence of ammonium ions in the moist paste of ammonium chloride. (iv) Explain whether you agree with the following statement.
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‘Zinc-carbon cells cause more environmental problems than nickel-cadmium cells do.’ (v) Complete and balance the following half-equations for the reactions that occur at the electrodes of a nickel-cadmium cell. Cd + OH- → Cd(OH)2 NiO2 + H2O → Ni(OH)2 + OH(9 marks) Acid and alkalis : 2000 7. (a) The sour taste of citrus fruits is mainly due to citric acid which is a weak acid with the following structure:
CH2CO2H HO C CO2H
(i) What is the meaning of the term ‘weak acid’? CH2CO2H (ii) Given that citric acid is a tribasic acid, write a chemical equation for the complete neutralization of nitric acid by sodium hydroxide solution. (iii) In a titration experiment to determine the concentration of citric acid in orange juice, 25.0 cm3 of a sample of orange juice required 10.0 cm3 of 0.30 M sodium hydroxide solution to reach the end-point, at which the citric acid had been completely neutralized. (1) Given that the relative molecular mass of citric acid is 192, calculate the concentration of citric acid, in g dm-3, in the sample of orange juice. (2) State ONE assumption in your calculation in (1). (3) Given that the following materials and apparatus, briefly describe how a 0.30 M sodium hydroxide solution can be prepared. 1.20 M sodium hydroxide solution distilled water 25.0 cm3 pipette 50.0 cm3 volumetric flask, 100.0 cm3 volumetric flask and 250.0 cm3 volumetric flask (Only one of these flasks should be used.) (10 marks) 4. The mass of a sample of copper powder contaminated with copper(II) oxide is known. Describe how you would conduct an experiment to derermine the percentage by mass of copper powder in the sample. State the expected observation of the experiment. (Hint: You may use an acid in the experiment.) (9 marks) 8. (c) State whether each of the following statements is true or false. Explain your answer in each case. (ii) When concentrated sulphuric acid is diluted, water should be added slowly to the acid. (2 marks)
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