The Foundations of Chemistry

The definition of molarity specifies the

amount of solute per unit volume of

solution,whereas percent specifies

the amount of solute per unit mass of

solution.Molarity therefore depends

on temperature and pressure, whereas

percent by mass does not.

3-6 Concentrations of Solutions 105

Note that the answer is not 200. mL but considerably less because 1.00 mL of solution has a
mass of 1.16 grams; however, 172 mL of the solution has a mass of 200 g.

You should now work Exercise 58.

Molarity

Molarity (M), or molar concentration, is a common unit for expressing the concentrations
of solutions. Molarityis defined as the number of moles of solute per liter of solution:

molarity

To prepare one liter of a one molar solution, one mole of solute is placed in a one-
liter volumetric flask, enough solvent is added to dissolve the solute, and solvent is then
added until the volume of the solution is exactly one liter. Students sometimes make the
mistake of assuming that a one molar solution contains one mole of solute in a liter of
solvent.This is notthe case; one liter of solvent plusone mole of solute usually has a to-
tal volume of more than one liter. A 0.100 Msolution contains 0.100 mol of solute per
liter of solution, and a 0.0100 Msolution contains 0.0100 mol of solute per liter of solu-
tion (Figure 3-2).

number of moles of solute



number of liters of solution

See the Saunders Interactive

General Chemistry CD-ROM,

Screen 5.11, Preparing Solutions, Direct

Addition.

Figure 3-2 Preparation of 0.0100 Msolution of KMnO 4 , potassium permanganate. A
250.-mL sample of 0.0100 MKMnO 4 solution contains 0.395 g of KMnO 4 (1 mol158 g).
(a) 0.395 g of KMnO 4 (0.00250 mol) is weighed out carefully and transferred into a 250.-mL
volumetric flask. (b) The KMnO 4 is dissolved in water. (c) Distilled H 2 O is added to the
volumetric flask until the volume of solution is 250. mL. The flask is then stoppered, and
its contents are mixed thoroughly to give a homogeneous solution.

(a) (b) (c)