Solutions

Percent by volume usually describes solutions in which both solute and solvent
are liquids. The percent by volume is the ratio of the volume of the solute
to the volume of the solution expressed as a percent. The volume of the solution
is the sum of the volumes of the solute and the solvent. Calculations are
similar to those involving percent by mass.
volume of solute
Percent by volume = 1 fIt' X
vo ume
100
0 so u lOn
Rubbing alcohol is an aqueous solution of liquid isopropyl alcohol. The
label on a typical container, such as the one shown in Figure 15-12, usually
states that the rubbing alcohol is 70% isopropyl alcohol. This value is a percent
by volume. It tells you that 70 volumes ofisopropyl alcohol are dissolved
in every 100 volumes of solution. Because a solution's volume is the sum of
the volumes of solute and solvent, there must be 30 volumes of water (solvent)
in every 100 volumes of the rubbing alcohol.
PROBLEMS
11. What is the percent by volume of ethanol in a solution that contains
35 mL of ethanol dissolved in 115 mL of water
12. If you have 100.0 mL of a 30.0% aqueous solution of ethanol, what
volumes of ethanol and water are in the solution
13. What is the percent by volume of isopropyl alcohol in a solution that
contains 24 mL of isopropyl alcohol in 1.1 L of water
Molarity
As you have learned, percent by volume and percent by mass are only two
of the commonly used ways to quantitatively describe the concentrations of
liquid solutions. One of the most common units of solution concentration is
molarity. Molarity (M) is the number of moles of solute dissolved per liter
of solution. Molarity also is known as molar concentration. The unit M is read
as molar. A liter of solution containing one mole of solute is a 1M solution,
which is read as a one molar solution. A liter of solution containing 0.1 mole
of solute is a O.lM solution.
To calculate a solution's molarity, you must know the volume of the solution
and the amount of dissolved solute.
Molarit (M) = ~oles of sol~te
y liters of solution
:. RUBEFACIENT JTOPICAL ANTI""CROE1~l
32 FL OZ (1 an 946 mL
For example, suppose you need to calculate the molarity of 100.0 rnL of an
aqueous solution containing 0.085 mole of dissolved potassium chloride
(KCl). You would first convert the volume of the solution from milliliters to
liters using the conversion factor 1 L = 1000 rnL.
Figure 15·12
The composition of this isopropyl
alcohol is given in percent by volume,
which is often expressed as
% (v/v). What does each v in the
expression 70% (vJv) refer to
(l00)Rl::) (lOJo~,)= 0.1000 L
Then, to determine the molarity, you would divide the number of moles of
solute by the solution volume in liters.
0.085 mol KCI
0.1000 L solution
0.8tmol = 0.85M
464 Chapter 15 Solutions