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A Solution strength by parts sea water is 35‰. Note the ‰ symbol, which means parts per
thousand just as % means parts per hundred. The equivalent
percent measure for salinity is 3.5%, which equals 35‰.
+
EXAMPLE 11.1
Vinegar that is prepared for table use is a mixture of acetic acid in
2 parts 98 parts 2 parts in 100
solute solvent solution water, usually 5.00% by weight. How many grams of pure acetic acid
are in 25.0 g of vinegar?

B Solution strength by percent (volume)
SOLUTION
The percent by weight is given (5.00%), the mass of the solution is given
(25.0 g), and the mass of the solute (CH 3 COOH) is the unknown. The
+ relationship between these quantities is found in equation 11.2, which
can be solved for the mass of the solute:
2 volume 98 volume 100 volume
solute solvent solution % solute = 5.00%
m solution = 25.0 g
m solute = ?
C Solution strength by percent (weight)
_
m solute

× 100% solution = % solute
m solution
∴
(m solution )(% solute)
__

m solute =
100% solution
(m solution )(% solute)
__

=

100% solution
(25.0 g)(5.00)
+ __
s
= olute
100
= 1.25 g solute
2 weights of 98 weights of 100 weights of
solute solvent solution
EXAMPLE 11.2
FIGURE 11.7 Three ways to express the amount of solute in A solution used to clean contact lenses contains 0.002% by volume of
a solution: (A) as parts (e.g., parts per million), this is 2 parts per thimerosal as a preservative. How many L of this preservative are needed
100; (B) as a percent by volume, this is 2 percent by volume; to make 100,000 L of the cleaning solution? (Answer: 2 L)
(C) as percent by weight, this is 2 percent by weight.
hydrogen peroxide is in 100 oz of solution. Since weight is pro-
Recall from chapter 10 that a mole is a measure of amount
portional to mass in a given location, mass units such as grams
used in chemistry. One mole is defined as the amount of a sub-
are sometimes used to calculate a percent by weight. The rela-
stance that contains the same number of elementary units as
tionship for percent by weight (using mass units) is
there are atoms in exactly 12 grams of the carbon-12 isotope.
__ The number of units in this case is called Avogadro’s number,
mass of solute

× 100% solution = % solute

mass of solution which is 6.02 × 10 —a very large number. This measure can
23
or
_
m solute
× 100% solution = % solute

m solution
equation 11.2
Both percent by volume and percent by weight are defined
as the volume or weight per 100 units of solution because per-
cent means parts per hundred. The measure of dissolved salts
in seawater is called salinity. Salinity is defined as the mass of
salts dissolved in 1,000 g of solution. As illustrated in Figure 11.8, FIGURE 11.8 Salinity is a measure of the amount of salts dis-
evaporation of 965 g of water from 1,000 g of seawater will solved in 1 kg of solution. If 1,000 g of seawater were evaporated,
leave an average of 35 g of salts. Thus, the average salinity of the 35.0 g of salts would remain as 965.0 g of water left.
282 CHAPTER 11 Water and Solutions 11-8