Concept explainers
A glass of water initially at pH 7.0 is exposed to dry air at sea level at
TABLE 21.1 Composition of Dry Air at Sea Level |
|
Gas |
Composition (% by Volume) |
|
78.03 |
|
20.99 |
|
0.94 |
|
0.033 |
|
0.0015 |
|
0.000524 |
|
0.00014 |
|
0.000006 |
Interpretation:
The pH of water at the equilibrium state is to be calculated.
Concept introduction:
Henry’s law is a gas law that states that the partial pressure of a gas and the concentration of a dissolved gas are directly proportional to each other.
Henry’s law is represented by the following expression:
Here,
is the mole fraction and
Equilibrium constant is expressed as follows:
Answer to Problem 68AP
Solution: pH is
Explanation of Solution
The partial pressure of carbon dioxide is calculated as follows:
Here,
is the mole fraction of
Substitute the value of
(from table
) and
Therefore, the partial pressure of carbon dioxide is
The concentration of
in water is calculated using Henry’s law as follows:
Here,
Substitute the values of
and
in the above equation:
Therefore, the concentration of
Let all of the dissolved
of carbonic acid is formed. Carbonic acid is a weak acid.
The equilibrium expression for carbonic acid in water is as follows:
The equilibrium constant is
).
The
Substitute the values of equilibrium concentration in the above equation:
This is a quadratic equation, by solving:
The pH of water at equilibrium is
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Chapter 21 Solutions
Chemistry
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