Question

$50mL$ of $0.1MHCl$ and $50mL$ of $0.2MNaOH$ are mixed. The pH of the resulting solution is

• A. $1.30$
• B. $4.2$
• C. $12.70$
• D. $11.70$

Answer 1

The correct option is C $12.70$

$50mL$ of $0.1MHCl=50\times 0.1$ millimoles $=5$ millimoles.
$50mL$ of $0.2NaOH=50\times 20$ millimoles $=10$ millimoles.
Millimoles of $NaOH$ left after neutralization $=5$.
Volume of solution $=100mL$.
$\therefore$ Molar conc. of $NaOH=\frac{5}{100}=5\times {10}^{-2}M$
$pOH=-log(5\times {10}^{-2})$
$=log\frac{1}{5\times {10}^{-2}}=log20=1.301$
$pH=14-1.301=12.699=12.70$

Hence, Option "C" is the correct answer.