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General Chemistry Principles, Patterns, and Applications, 2011

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21. Dissolve the mixture of A <strong>and</strong> B in a solvent in which they are both soluble when hot <strong>and</strong> relatively insoluble<br />

when cold, filter off any undissolved B, <strong>and</strong> cool slowly. Pure A should crystallize, while B stays in solution. If<br />

B were less soluble, it would be impossible to obtain pure A by this method in a single step, because some of<br />

the less soluble compound (B) will always be present in the solid that crystallizes from solution.<br />

13.3 Units of Concentration<br />

L E A R N I N G O B JE C T I V E<br />

1. To describe the concentration of a solution in the way that is most appropriate for a<br />

particular problem or application.<br />

There are several different ways to quantitatively describe the concentration of a solution. For example, molarity<br />

was introduced in as a useful way to describe solution concentrations for reactions that are carried out in solution.<br />

Mole fractions, introduced in , are used not only to describe gas concentrations but also to determine the vapor<br />

pressures of mixtures of similar liquids. Example 4 reviews the methods for calculating the molarity <strong>and</strong> mole fraction<br />

of a solution when the masses of its components are known.<br />

E X A M P L E 4<br />

Commercial vinegar is essentially a solution of acetic acid in water. A bottle of vinegar has 3.78 g of acetic<br />

acid per 100.0 g of solution. Assume that the density of the solution is 1.00 g/mL.<br />

a. What is its molarity?<br />

b. What is its mole fraction?<br />

Given: mass of substance <strong>and</strong> mass <strong>and</strong> density of solution<br />

Asked for: molarity <strong>and</strong> mole fraction<br />

Strategy:<br />

A Calculate the number of moles of acetic acid in the sample. Then calculate the number of liters of<br />

solution from its mass <strong>and</strong> density. Use these results to determine the molarity of the solution.<br />

B Determine the mass of the water in the sample <strong>and</strong> calculate the number of moles of water. Then<br />

determine the mole fraction of acetic acid by dividing the number of moles of acetic acid by the total<br />

number of moles of substances in the sample.<br />

Solution:<br />

Saylor URL: http://www.saylor.org/books<br />

Saylor.org<br />

1201

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