Question

The pH of the solution obtained by mixing equal volumes of solution of $pH=5$ and $pH=3$ of the same electrolyte is:

• A. $3.3$
• B. $4.0$
• C. $4.5$
• D. $2.0$

Answer 1

The correct option is A $3.3$

Sample-1:

$pH=5$

$pH=-log\left[H^{+}\right]=5$

$\Rightarrow \left[H^{+}\right]={10}^{-5}M=\left(M_1\right)$

$V_1=V_0$

Sample2:

$pH=3$

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

$\Rightarrow \left[H^{+}\right]={10}^{-3}M=\left(M_2\right)$

$V_2=V_0$

The concentration of the mixture of two solutions will be

$MV=M_1{V}_1+M_2{V}_2$

$M(V_0+V_0)=\left({10}^{-5}\right)V_0+\left({10}^{-3}\right)V_0$

$\Rightarrow M=\frac{{10}^{-5}+{10}^{-3}}{2}=0.000505$

$pH=-log\left[H^{+}\right]=-log\left(0.000505\right)=-(-3.29)=3.3$