Question

$1.0M$ solution of weak acid $HA$ is only $0.5$% ionized. If $2ml$ of 1 M solution of $HA$ is mixed with $30ml$ water, the degree of dissociation of the acid in the resulting solution and $\left[{OH}^{-}\right]$ ion concentration of the solution will be respectively.

• A. $0.02and1.25\times {10}^{-3}M$
• B. $0.08and3.5\times {10}^{-3}M$
• C. $0.04and3\times {10}^{-4}M$
• D. $0.02and8\times {10}^{-12}M$

Answer 1

Answer: (D)

$0.02and8\times {10}^{-12}M$

$HA\rightleftharpoons H^{+}+A^{-}$

$K_a=\frac{{\left(0.005\right)}^2}{1-0.005}=\frac{{\left(\frac{1}{16}\right)}^2}{\frac{1}{16}(1-\alpha )}$

$0.000025=\frac{1}{16}{\alpha }^2$

$\alpha =0.02$

$\left[H^{+}\right]=\frac{1}{16}\times 0.02$

$\left[{OH}^{-}\right]=\frac{{10}^{-14}}{\left[H^{+}\right]}=8\times {10}^{-12}$
Option D is correct.