Weak acid pH practice problems

Chemistry Unit 8. Try each one, then open the worked solution. The Credit Sprint app generates unlimited new versions with fresh numbers.

Problem 1.

What is the pH of a 0.05 M solution of a weak acid with Ka = 1.8 × 10⁻⁵? (Use the approximation [H⁺] = √(Ka·C).)

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[H⁺] ≈ √(1.8 × 10⁻⁵ × 0.05) = 0.000949 M; pH = 3.02.

Problem 2.

What is the pH of a 0.1 M solution of a weak acid with Ka = 7.2 × 10⁻⁴? (Use the approximation [H⁺] = √(Ka·C).)

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[H⁺] ≈ √(7.2 × 10⁻⁴ × 0.1) = 0.00849 M; pH = 2.07.

Problem 3.

What is the pH of a 0.2 M solution of a weak acid with Ka = 7.2 × 10⁻⁴? (Use the approximation [H⁺] = √(Ka·C).)

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[H⁺] ≈ √(7.2 × 10⁻⁴ × 0.2) = 0.012 M; pH = 1.92.

Problem 4.

What is the pH of a 0.2 M solution of a weak acid with Ka = 1.8 × 10⁻⁵? (Use the approximation [H⁺] = √(Ka·C).)

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[H⁺] ≈ √(1.8 × 10⁻⁵ × 0.2) = 0.0019 M; pH = 2.72.

Problem 5.

What is the pH of a 0.1 M solution of a weak acid with Ka = 7.2 × 10⁻⁴? (Use the approximation [H⁺] = √(Ka·C).)

Show solution

[H⁺] ≈ √(7.2 × 10⁻⁴ × 0.1) = 0.00849 M; pH = 2.07.

Problem 6.

What is the pH of a 0.2 M solution of a weak acid with Ka = 7.2 × 10⁻⁴? (Use the approximation [H⁺] = √(Ka·C).)

Show solution

[H⁺] ≈ √(7.2 × 10⁻⁴ × 0.2) = 0.012 M; pH = 1.92.