Solutions

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HOW TO MAKE STANDARD SOLUTIONS FOR CHEMISTRY
Phillip Bigelow Chemists make two common types of "standard solutions": Molar solutions Normal solutions Both of these solutions are concentrations (or “strengths”) of a particular component (solute) that is dissolved in a solvent. Making a Molar solution usually involves fewer mental steps than does making a Normal solution. Below, I will describe both methods. ____________________________ PART 1: MAKING MOLAR (M) SOLUTIONS A 1 Molar solution (1M) contains 1 mole of solute dissolved in a solution totaling 1 liter. If you use water as the solvent, it must be distilled and deionized. Do not use tap water. A mole is the molecular weight (MW) expressed in grams (sometimes referred to as the ‘gram molecular weight’ (gMW) of a chemical). Thus, 1 M = 1 gMW of solute per liter of solution. Problem: How much sodium chloride is needed to make 1 liter of an aqueous 1 M solution? Answer: First, we calculate the molecular weight (MW) of sodium chloride. Checking the Periodic Table of Elements, we find that the atomic weight of sodium (Na) is 23 and the atomic weight of chlorine (Cl) is 35.5. Therefore, the molecular weight of sodium chloride (NaCl) is: Na (23) + Cl (35.5) = 58.5 grams/mole. To make a 1M aqueous solution of NaCl, dissolve 58.5 grams of NaCl in some distilled deionized water (the exact amount of water is unimportant; just add enough water to the flask so that the NaCl dissolves). Then add more water to the flask until it totals 1 liter. You’re done.

Similarly, a 2M solution of sodium chloride contains 117 grams of the salt (2 × 58.5 grams), topped-off with water to the one liter mark. Likewise, a 0.1M solution of sodium chloride contains 5.85 grams (58.5 grams/10) of the salt, topped-off with water to the one liter mark. MAKING MOLAR SOLUTIONS FROM CONCENTRATED AQUEOUS ACIDS AND BASES Making a standard molar solution from aqueous acids or bases is a bit more involved than making a standard molar solution from a solid chemical. This is because nearly all liquid acids, no matter how concentrated they are, are already diluted to some extent with water (for instance, pure HCl is a gas, not a liquid, and it is rarely sold in its pure form). Let’s use sulfuric acid as our first example. Problem: Make one liter of a 1 Molar (1M) aqueous solution of H 2SO4. Answer: The first step is to read the label on the bottle of the H 2SO4 reagent. The label will tell you it's molarity. Although there are a variety of concentrations of acids, concentrated H2SO4 often comes from the factory at a 18.0 Molar concentration (Table 1). This means that there are 18 moles of H2SO4 in each liter of solution (note: do not rely on Table 1; always check the label on the bottle). You need to make a much more diluted solution, so you will add one mole of the concentrated reagent to a fresh batch of water. Your task is to calculate how many milliliters of reagent contain one mole of the acid. We know from reading the label on the bottle (“18.0 Molar”) that one liter of reagent contains 18.0 moles of H2SO4. This means that 1 ml of reagent contains 0.018 moles of H2SO4. Therefore, 1 ml x ml _________ = ______. 0.018 moles 1 mole Solving for x, we find that we need 55.6 ml of H 2SO4 reagent.

Therefore, we slowly add 55.6 ml of the H 2SO4 reagent to about 500 ml of distilled deionized water, and then we top it off with more water to exactly the "1-liter" mark on the flask. solution. This procedure works You have successfully made a 1 Molar H2SO4 similarly with aqueous bases.

Caution: Never add water into a large volume of concentrated acid! You risk creating an explosion! The rule is: “Acid into water = you’re doing what ya oughta.” “Water into acid = you might get blasted!” Therefore, always add a smaller volume of acid into a larger volume of water. Table 1 TYPICAL CONCENTRATIONS OF CONCENTRATED ACIDS AND BASES (as written on the labels of their containers) ACID/BASE NAME Acetic acid Ammonium hydroxide (aqueous ammonia) Hydrochloric acid Nitric acid (HNO3) Phosphoric acid Sulfuric acid WT% 99.7% 28% 37% 70% 85% 96% DENSITY (sp. gr) MOLARITY (g/ml) 1.05 g/ml 0.89 g/ml 1.18 g/ml 1.40 g/ml 1.69 g/ml 1.84 g/ml 17.4 14.6 12.0 15.6 14.7 18.0

_______________________________________ PART 2: MAKING NORMAL SOLUTIONS (N) Compared to making Molar solutions, making Normal solutions can be a bit confusing. Aqueous solutions of acids and bases are often described in terms of their normality rather than their molarity. In order to properly make a Normal solution, the worker must understand the difference between a pure reagent and a diluted reagent. A "1 Normal" solution (1 N) contains 1 “gram equivalent weight” (gEW) of solute, topped-off to one liter of solution. The gram equivalent weight is equal to the solute’s molecular weight, expressed as grams, divided by the valence (n) of the solute: Equivalent weight (EW) = molecular weight _____________ n

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