Question

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

• A. $2.4$
• B. $7.29$
• C. $6.71$
• D. $9.81$

Answer 1

Answer: (A)

$2.4$

$pH$ of $HCl=2$ $\therefore$ $\left[HCl\right]={10}^{-2}M$
$pH$ of $HCl=4$ $\therefore$ $\left[HCl\right]={10}^{-4}M$

So total conc. of hydrogen ion, $\left[HCl\right]=\frac{{10}^{-2}\times 200}{500}+\frac{{10}^{-4}\times 300}{500}$

$=4\times {10}^{-3}+6\times {10}^{-5}=4.06\times {10}^{-3}M$

$\therefore pH=-\log \left[H^{+}\right]=-\log 4.06\times {10}^{-3}$

$=2.3914$