Chapter 16 Text
Chapter 16 Text
Chapter 16 Text
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<strong>16</strong>.4 The pH Scale 621<br />
SAMPLE EXERCISE <strong>16</strong>.4<br />
Calcinate the values of [H+] and [OH"] in a neutral solution at 25°C.<br />
Solution<br />
Analyze: We are asked to determine the concentrations of hydronium and hydroxide<br />
ions in a neutral solution at 25 °C.<br />
Plan: We will use Equation <strong>16</strong>.<strong>16</strong> and the fact that, by definition, [H+] = [OH~] in a<br />
neutral solution.<br />
Solve: We will represent the concentration of [H+] and [OH~] in neutral solution with x.<br />
This gives<br />
[H+][OH-] - (*)(*) *= 1.0 X 1(T14<br />
x2 = 1,0 x 10~14<br />
1-7<br />
x = 1.0 X l [H+].<br />
(b) In this instance<br />
[OH ] 1.8 x 10<br />
This solution is acidic because [H+] > [OH~].<br />
=<br />
x 1Q_12M<br />
a-<br />
m<br />
fire<br />
a-<br />
3d<br />
I*<br />
ns<br />
of<br />
PRACTICE EXERCISE<br />
Calculate the concentration of OH~(aq) in a solution in which (a) [H+] = 2 X 10~6M;<br />
(b) [H+] = [OH"]; (c) [H+] = 100 x [OH~].<br />
Answers: (a) 5 X 10~9 M; (b) 1.0 X 10~7 M; (c) 1.0 x 10~8 M<br />
<strong>16</strong>.4 The pH Scale<br />
The molar concentration of H+(aq) in an aqueous solution is usually very small.<br />
For convenience, we therefore usually express [H+] in terms of pH, which is the<br />
negative logarithm in base 10 of [H+].*<br />
pH = -log[H+] [<strong>16</strong>.17]<br />
"Because [H*] and [H3O+] are used interchangeably, you might see pH defined as -log[H3O ].<br />
In chemistry, pH is one of the few<br />
logarithmic functions that are<br />
actually defined in terms of the<br />
common (base 10} logarithm.<br />
Addison Ault, "Do pH in Your Head,<br />
}. Chem. Educ., Vol. 76, 1999,<br />
936-938.