Question

The dissociation constant of a weak acid HA is $1\times {10}^{-5}$. Its concentration is 0.1 M. pH of that solution is:

• A. 1
• B. 2
• C. 3
• D. 5

Answer 1

Answer: (C)

3

For a weak acid, $\left[H^{+}\right]=\sqrt{K_a\times c}$.

The dissociation constant is $1\times {10}^{-5}$ and the concentration is 0.1 M.

Substitute values in the above expression,

$\left[H^{+}\right]=\sqrt{1\times {10}^{-5}\times 0.1}=1\times {10}^{-3}$.

$pH=-log\left[H^{+}\right]=-log{10}^{-3}=3$.