Chapter 10 Solutions

10.2

THE SOLUTION PROCESS There is usually a limit to how much solute can

dissolve in a given amount of solvent at a

given temperature. This limit is called the

solubility

of the solute in that solvent at that

temperature. Solutions in which the concentra

tion of the solute has reached this level are

said to be

saturated

. Substances are often classified as so

luble or insoluble in a solvent,

but solubilities vary continuously. In this te

xt, substances with solubilities of 0.1 M or

greater are considered to be

soluble

; those with solubilities between 0.1 and 0.01 M are

referred to as

moderately soluble

, and those with solubilities below 0.01 M are called

insoluble

. Thus, a saturated solution of NaCl in water is about 7 M, so NaCl is soluble in

water; a saturated solution of MgSO

is 0.015 M, so MgSO 4

is moderately soluble in 4

water; a saturated solution of s

ilver chloride is only about 10

-5 M, so AgCl is considered to

be insoluble in water. Two substances, such as water and ethanol that are soluble in one another in all proportions are said to be

miscible

.

Example 10.3

Given the following solubilities of some fluorides in 100 mL of water at 25

oC,

classify each as soluble, moderately soluble, or insoluble.

a) 2.56 g NiF

(M 2

= 96.7 g/mol) m

Molarity is moles of solute per liter of solu

tion, using the factor-label method we obtain

2

2.56 g NiF

100 mL

1000 mL×

×

2 2

1 mol NiF

1 L

96.7 g NiF

2

0.265 mol NiF

=

= 0.265 M

1 L

The solubility in water is greater

than 0.1 M. We conclude that NiF

is 2

soluble

in water.

b) 0.075 g CuF

(M 2

= 102 g/mol) m

Proceed as in Part A.

2

0.075 g CuF

100 mL

1000 mL×

×

2 2

1 mol CuF

1 L

102 g CuF

×

×

-3

-3

2

7.4 10 mol CuF

=

= 7.4 10 M

1 L water

The solubility of CuF

is less than 0.01 M, so we conclude that CuF 2

is insoluble 2

in water.

c) 0.417 g S

F 2

(M 2

= 102 g/mol) m 22

0.417 g S F

100 mL

1000 mL×

×

(^2222)
1 mol S F
1 L
102 g S F
22
0.0409 mol S F

= 0.0409 M
1 L water
The solubility of S
F 2
is between 0.01 M and 0.1 M, so it is 2
moderately soluble
in water.
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