Question

The hydrogen ion concentration of 0.5M aqueous solution of $HF(K_a=2\times {10}^{-4})$ is:

• A. $\text{1}{\text{0}}^{-2}M$
• B. $2\times {10}^{-3}M$
• C. $4\times {10}^{-3}M$
• D. ${10}^{-5}M$

Answer 1

Answer: (A)

$\text{1}{\text{0}}^{-2}M$

$HF\rightleftharpoons H^{+}+F^{-}$

Initial 0.5 0 0

Eq. $0.5-\alpha$ $\alpha$ $\alpha$

$K_a=\frac{\left[H^{+}\right]\left[F^{-}\right]}{\left[HF\right]}$

$2\times {10}^{-4}=\frac{\left[\alpha \right]\left[\alpha \right]}{[0.5-\alpha ]}$

$\alpha =1\times {10}^{-2}$

$\left[H^{+}\right]=\alpha ={10}^{-2}$ M

Hence, option $A$ is correct.