Question

The $\left[H^{+}\right]$ of a resulting solution that is $0.01$M acetic acid $(K_a=1.8\times {10}^{-5})$ and $0.01$M in benzoic acid $(K_a=6.3\times {10}^{-5})$:

• A. $9\times {10}^{-4}$
• B. $81\times {10}^{-4}$
• C. $9\times {10}^{-5}$
• D. $2.8\times {10}^{-3}$

Answer 1

Answer: (A)

$9\times {10}^{-4}$

$\alpha$ is negligible w.r.t. 1 for both acid so
$\left[H^{+}\right]=\sqrt{K_{a1}{C}_1+K_{a2}{C}_2}$
$\left[H^{+}\right]=\sqrt{1.8\times {10}^{-5}\times 0.01+6.3\times {10}^{-5}\times 0.01}$
$\left[H^{+}\right]=\sqrt{81\times {10}^{-8}}=9\times {10}^{-4}$