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Solution chemistry and aqueous equilibria 69

4.2 Solution concentrations
The amount of solute present in a given quantity of solvent (or total
solution) is called the concentration of the solution. The concentration
of a solution may be given as a mass/total mass percentage. For
example, a 3% NaCl solution by mass means 3 g of NaCl dissolved in
97 g of pure water. It may also be given as a volume/total volume
percentage. Therefore, a 20% alcohol-water solution by volume is 20
mL of alcohol in 80 mL of pure water. Mass/volume percentage may
also be used. If a solution concentration is given as simply a percent­
ag , it may be assumed it is a mass/total mass percentage.
e
The preferred unit for the concentration of a solution in the SI
system is molar concentration (or molarity) which is defined by

Number of moles of solute
Molar concentration = ( 4 . 1 )
N um er o 1ters o so ut1on
f 1
f r
b
·
2
The units of molarity are mol L 1 (or M for short). For example, a . 5
-

M solution contains . 5 gram moles of solute n every liter f solution.
2
i
o
Since the volume of a solution changes with temperature, so will the
molarity, even though the amount of solute remains the same. If,
however, the concentration of a given solution is expressed as moles
of solute per kilogram of solvent (called molal concentration or mo­
lality), it will be independent of temperature. The units of molality are
mol kg- 1 (or , for short).
m
The concentration of a particular solute i in a solution can also be
expressed as a mole fraction
Number of moles of i
"' - ( 4 . 2 )
-
; Total number of moles of all compounds in the solution
where i/J; is dimensionles .
s
Exercise 4. 1 . Assuming that dry air consists of just molecular nitro­
gen, molecular oxygen, and atomic argo , and that these three gases
n
contribute 75. 5 , 23 . 2 , and . 30 kg to 1 00 kg of air, calculate their mole
1
fractions in air.
Solution. Since the molecular weights of nitrogen, oxygen, and
argon are 28.0, 32.0, and 39.9, respectively, the numbers of moles of
nitroge , oxygen, and argon in 1 0 0 kg of dry air are
n
75. 5 x 0 1 3 23. 3 x 1 0 3 - 725 and l . 3 0 x 1 0 3 = 3 2 . 6 , respectively.
28.0 = 27000, 32.0 39 9
_