Question

The pH of the solution obtained by mixing 100 ml of a solution of pH = 3 with 400 ml of a solution of pH = 4 is :

• A. $3-log2.8$
• B. $7-log2.8$
• C. $4-log2.8$
• D. $5-log2.8$

Answer 1

The correct option is C $4-log2.8$

The pH of the solution obtained by mixing 100 ml of a solution of pH 3 with 400 ml of a solution of pH 4 is
(a) 100 ml of a solution of pH = 3
$\left[H^{+}\right]={10}^{-pH}={10}^{-3}$ M
(b) 400 ml of a solution of pH = 4
$\left[H^{+}\right]={10}^{-pH}={10}^{-4}$ M
When two solutions are mixed,

$\left[H^{+}\right]=\frac{100\times {10}^{-3}+400\times {10}^{-4}}{100+400}=0.00028$ M $=2.8\times {10}^{-4}M$
$pH=-log\left[H^{+}\right]=-log(2.8\times {10}^{-4})=-\left(log2.8\right)-\left(log{10}^{-4}\right)=4-log2.8$

Hence, option (C) is the correct answer.