Solutions
Part II: Units of
Concentration
(loosely from Jespersen Chap. 13 Sec 5)
Dr. C. Yau
Spring 2014
1
Units of Concentration
# mol solute
Molarity (M) M = molarity 
# L solution
Molality (m)
Mole fraction
# mol solute
m = molality 
# kg solvent
nsolute
nsolvent  nsolute
nsolvent
nsolvent  nsolute
& mole %
nsolute
x 100
nsolvent  n solute
Mass %, ppt, ppm, ppb
2
Percent by Mass
mass of solute
conc in % by mass =
x 100
mass of soln
mass of solute
x 1000
conc in ppt by mass =
mass of soln
mass of solute
6
x 10
conc in ppm by mass =
mass of soln
6
1 million = 1,000,000 = 10
mass of solute
x 109
conc in ppb by mass =
mass of soln
1 billion = 1,000,000,000 = 109
3
What does 15% w/w mean?
w/w means
weight of solute
weight of solution
15% w/w means
15 g solute
100 g solution
1 ppt means....?
1 g solute
1 mg solute
or
etc
1000 g soln
1000 mg soln
15 ppb
15 g solute
15 g solute
means....? 1,000,000,000 g soln or 109 g soln etc.
4
Example 13.2 p.598
Please don’t waste time copying the question. You
can get it later from your textbook.
Seawater is typically 3.5% sea salt and has a
density of about 1.03 g mL-1. How many grams
of sea salt would be needed to prepare enough
seawater solution to completely fill a 62.5 L
aquarium?
5
Molality vs Molarity
Know WHEN m is almost the same as M...under what
conditions? Ans. m = M for very dilute solutions
If we have a very dilute solution, such as
0.0005 M which is
0.0005 mol solute 0.0005 mol solute

= 0.0005 m
1 Kg water
1 L soln
In such a solution, there is very little solute
present, therefore 1 L soln = 1 L water.
 103 mL   1 g   1 Kg 
Mass of 1 L water =1 L 

  3  = 1 Kg
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 1 L   1 mL   10 g 
Ex 13.3 p.599
An experiment calls for a 0.150 m solution
of sodium chloride in water. How many
grams of NaCl would have to be dissolved
in 500.0 g of water to prepare a solution of
this molality?
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Interconversion of Units of
Concentration
Example 13.4 p. 601
What is the molality of 10.0% w/w aq NaCl?
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Example 13.5 p. 602
A certain supply of conc hydrochloric acid
has a concentration of 36.0% HCl. The
density of the solution is 1.19 g mL-1.
Calculate the molar concentration of HCl.
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p.629 #13.48
What is the molality of NaCl in a solution that
is 3.000 M NaCl, with a density of
1.07 g mL-1?
Be sure to learn how to convert between
different units of concentration.
Practice Exer 6-8 on p.600
Exer 9 & 10 on p.602
Exer 11 & 12 on p.603
Do as many as possible: p.629 #48-57.
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