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Solving Problems: A Chemistry Handbook
Chemistry: Matter and Change
185
SOLVING PROBLEMS:
A CHEMISTRY HANDBOOK
CHAPTER 18
At 298 K, the product of [H ϩ ] and [OH Ϫ ] always equals 1.0 ϫ 10 Ϫ14 .
Therefore, if the concentration of one of these ions increases, the
concentration of the other ion must decrease. The following example
problem shows how you can use K w to find either [H ϩ ] or [OH Ϫ ] if
the other concentration is known.
Example Problem 18-2
Using K w to Calculate [H ϩ ] and [OH Ϫ ]
At 298 K, the OH Ϫ ion concentration of an aqueous solution is
1.0 ϫ 10 Ϫ11 M. Find the H ϩ ion concentration in the solution and
determine whether the solution is acidic, basic, or neutral.
Write the ion product constant expression.
K w ϭ [H ϩ ][OH Ϫ ] ϭ 1.0 ϫ 10 Ϫ14
Divide both sides of the equation by [OH Ϫ ].
[H ϩ ] ϭ
Substitute the values for K w and [OH Ϫ ] and solve.
[H ϩ ] ϭ
ϭ1.0 ϫ 10 Ϫ3 M
[H ϩ ] Ͼ [OH Ϫ ], so the solution is acidic.
Practice Problems
6. Given the concentration of either hydrogen ion or hydroxide
ion, calculate the concentration of the other ion at 298 K and
state whether the solution is acidic, basic, or neutral.
a. [OH Ϫ ] ϭ 1.0 ϫ 10 Ϫ6 M
b. [H ϩ ] ϭ 1.0 ϫ 10 Ϫ7 M
c. [H ϩ ] ϭ 8.1 ϫ 10 Ϫ3 M
pH and pOH Because the concentrations of H ϩ ions are often
very small numbers, the pH scale was developed as a more convenient way to express H ϩ ion concentrations. The pH of a solution
equals the negative logarithm of the hydrogen ion concentration.
pH ϭ Ϫlog [H ϩ ]
▲
1.0 ϫ 10 Ϫ14
ᎏᎏ
1.0 ϫ 10 Ϫ11
K w
ᎏ
[OH Ϫ ]