Question

The pH of a 0.01 M solution of a weak acid HA is 4. Calculate its dissociation constant (Ka).

• A. $1.0 \times 10^{-6}$
• B. $1.0 \times 10^{-8}$
• C. $1.0 \times 10^{-4}$
• D. $1.0 \times 10^{-5}$

Hint:

For weak acids, use the approximation $[\text{H}^+] = \sqrt{Ka \cdot C}$ or $Ka = [\text{H}^+]^2 / C$.