Question

What will be the resultant $pH$ (only integer part) when $200mL$ of an aqueous solution of $HCl$ $(pH=2.0)$ is mixed with $300mL$ of an aqueous solution of $HCl$ $(pH=4.0)$?

Answer 1

For first HCl solution,
$pH$ of $HCl=2$ $\therefore$ $\left[HCl\right]={10}^{-2}M$
For second HCl solution
$pH$ of $HCl=4$ $\therefore$ $\left[HCl\right]={10}^{-4}M$
Hence, for the mixture,
$\left[HCl\right]=\frac{{10}^{-2}\times 200}{500}+\frac{{10}^{-4}\times 300}{500}$
$=4\times {10}^{-3}+6\times {10}^{-5}=4.06\times {10}^{-3}$
$pH=-\log \left[H^{+}\right]=-\log 4.06\times {10}^{-3}$
$=2.3914$
Hence, the pH of the solution is 2.3914.