Question

Two solutions are mixed as given below:
$55\, mL \frac{ M }{10} HCl +45\, mL \frac{ M }{10} NaOH$
What is the $pH$ of the resulting solution?

Answer 1

$H ^{+}=5.5 \times 10^{-3} \,mol , OH ^{-}=4.5 \times 10^{-3} \,mol$
Excess $H ^{+}=1 \times 10^{-3} \,mol$
Total volume $=0.055+0.045=0.1\, L$
$\left[ H ^{+}\right]=\frac{1 \times 10^{-3}}{0.1}=1 \times 10^{-2} M$
$pH =-\log _{10}\left[ H ^{+}\right]=-\log _{10}\left(1 \times 10^{-2}\right)=2$