Question

What will be the resultant pH when $200mL$ of an aqueous solution of $HCl$ (pH=2) is mixed with $300mL$ of an aqueous solution of $NaOH$ (pH=12)?

Answer 1

pH of $HCl=2$ $\therefore \left[HCl\right]={10}^{-2}M$
pH of $NaOH=12$ $\therefore \left[NaOH\right]={10}^{-2}M$
$HCl+NaOH\to NaCl+H_{2}O$

meq. before reaction $200\times {10}^{-2}=2$ $300\times {10}^{-2}=3$
meq. after reaction $0$ $100\times {10}^{-2}=1$
$\left[{OH}^{-}\right]$ after reaction $=\frac{1}{500}=2\times {10}^{-3}M$
$pOH=-\log \left[{OH}^{-}\right]=-\log 2\times {10}^{-3}=2.7$
$pH=14-2.7=11.3$