Page 230 - Theory and Problems of BEGINNING CHEMISTRY

CHAPTER 15

Solutions

15.1. QUALITATIVE CONCENTRATION TERMS
Solutions are mixtures, and therefore do not have definite compositions. For example, in a glass of water it is
possible to dissolve one teaspoonful of sugar or two or three or more. However, for most solutions there is a limit to
how much solute will dissolve in a given quantity of solvent at a given temperature. The maximum concentration
of solute that will dissolve in given quantity of solvent is called the solubility of the solute. Solubility depends
on temperature. Most solids dissolve in liquids more at high temperatures than at low temperatures, while gases
dissolve in cold liquids better than in hot liquids.
A solution in which the concentration of the solute is equal to the solubility is called a saturated solution.
If the concentration is lower, the solution is said to be unsaturated. It is also possible to prepare a supersatu-
rated solution, an unstable solution containing a greater concentration of solute than is present in a saturated
solution. Such a solution deposits the excess solute if a crystal of the solute is added to it. It is prepared by
dissolving solute at one temperature and carefully changing the temperature to a point where the solution is
unstable.

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EXAMPLE 15.1. A solution at 0 C contains 119 g of sodium acetate per 100 g of water. If more sodium acetate is added,
it appears not to dissolve, and no sodium acetate appears to crystallize from solution either. Describe the following solutions
as saturated, unsaturated, or supersaturated. (a) 105 g sodium acetate in 100 g water at 0 C. (b) 142 g sodium acetate in 100 g
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water at 0 C. (c) 1.19 g sodium acetate in 1.00 g water at 0 C.
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Ans. (a) The solution is unsaturated; more solute could dissolve. (b) The solution is supersaturated; only 119 g is stable
in water at this temperature. (c) The solution is saturated; the concentration is the same as that given in the statement
of the problem.

EXAMPLE 15.2. How can the solution described in Example 15.1b be prepared?
Ans. The 142 g of sodium acetate is mixed with 100 g of water and heated to nearly 100 C (where 170 g of solute would
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dissolve). The mixture is stirred until solution is complete, and then the solution is cooled until it gets to 0 C. The
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solute would crystallize if it could, but this particular solute has difficulty doing so, and thus a supersaturated solution
is formed. Adding a crystal of solid sodium acetate allows the excess solute to crystallize around the solid added,
and the excess solute precipitates out of the solution, leaving the solution saturated.

15.2. MOLALITY
Molarity (Chap. 11) is defined in terms of the volume of a solution. Since the volume is temperature-
dependent, so is the molarity of the solution. Two units of concentration that are independent of temperature
are introduced in this chapter. Molality is defined as the number of moles of solute per kilogram of solvent in
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