Question

The hydrogen ion concentration of a $0.006$ M benzonic acid solution is $(K_a=6\times {10}^{-5})$

• A. $0.6\times {10}^{-4}M$
• B. $6\times {10}^{-4}M$
• C. $6\times {10}^{-5}M$
• D. $3.6\times {10}^{-4}M$

Answer 1

The correct option is B $6\times {10}^{-4}M$

Initial concentration $=0.006M\Rightarrow K_a=6\times {10}^{-5}$ equilibrium reaction.

$\begin{array}{ccccc}{C}_6{H}_{5}COOH& \rightleftharpoons & C_6{H}_5{COO}^{-}& +& H^{+}\\ C& & O& & Oinitial\\ C(1-\alpha )& & C\alpha & & C\alpha \end{array}$

$K_a=C{\alpha }^2(\because 1-\alpha \approx 1)$

$\alpha =\sqrt{\frac{K_a}{C}};\alpha =\sqrt{\frac{6\times {10}^{-5}}{0.006}};\alpha ={10}^{-1}$

$\because \left[H^{+}\right]=C\alpha =0.006\times {10}^{-1}=6\times {10}^{-4}M$.