General Chemistry Principles, Patterns, and Applications, 2011

E X A M P L E 1 6
a. What is the pH of a 2.1 × 10 −2 M aqueous solution of HClO 4?
b. The pH of a vinegar sample is 3.80. What is its hydrogen ion concentration?
Given: molarity of acid or pH
Asked for: pH or [H + ]
Strategy:
Using the balanced chemical equation for the acid dissociation reaction and Equation 4.41 or 4.40,
determine [H + ] and convert it to pH or vice versa.
Solution:
a. HClO 4 (perchloric acid) is a strong acid, so it dissociates completely into H + −
ions and ClO 4 ions:
HClO 4(l) → H + (aq) + ClO 4
− (aq)
The H + ion concentration is therefore the same as the perchloric acid concentration. The pH of the
perchloric acid solution is thus
pH = −log[H + ] = −log(2.1 × 10 −2 ) = 1.68
The result makes sense: the H + ion concentration is between 10 −1 M and 10 −2 M, so the pH must be
between 1 and 2.
Note: The assumption that [H + ] is the same as the concentration of the acid is valid for only strong
acids. Because weak acids do not dissociate completely in aqueous solution, a more complex
procedure is needed to calculate the pH of their solutions, which we will describe in Chapter 16
"Aqueous Acid–Base Equilibriums".
b. We are given the pH and asked to calculate the hydrogen ion concentration. From Equation 4.40,
10 −pH = [H + ]
Thus [H + ] = 10 −3.80 = 1.6 × 10 −4 M.
Exercise
a. What is the pH of a 3.0 × 10 −5 M aqueous solution of HNO 3?
b. What is the hydrogen ion concentration of turnip juice, which has a pH of 5.41?
Answer:
a. pH = 4.52
b. [H + ] = 3.9 × 10 −6 M
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