pH of a weak acid solution
What is the pH of a 0.1 M solution of a weak acid with Ka = 1 × 10⁻⁵?
Answer B: explanation for every choice
A
This treats the acid as strong and fully dissociated.
B
Correct. [H⁺] ≈ √(Ka·C) = √(10⁻⁶) = 10⁻³ M, so pH = 3.
C
5 is the pKa. pH equals pKa only in a buffer with equal acid and base.
D
This takes −log(Ka·C) without the square root.
TakeawayWeak acid: [H⁺] ≈ √(KaC), so pH ≈ ½(pKa − log C).
Equation usedpH of a weak acid
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