Question

By adding $20mL0.1NHCl$ to $20mL0.001NKOH$, the pH of the obtained solution will be

• A. 2
• B. 1.3
• C. 0
• D. 7

Answer 1

Answer: (B)

$20mL$ of $0.1NHCl=\frac{0.1}{1000}\times 20geq$
$=2\times 10^{-3}geq$
$20mL$ of $0.001KOH=\frac{0.001}{1000}\times 20geq$
$=2\times 10^{-5}geq$
$\therefore HCl$ left unneutralised
$=2\left(10^{-3}-10^{-5}\right)$
$=2\times 10^{-3}(1-0.01)$
$=2\times 0.99\times 10^{-3}$
$=1.98\times 10^{-3}geq$.
Volume of solution $=40mL$
$\therefore [HCl]=\frac{1.98\times 10^{-3}}{40}\times 1000M$
$=4.95\times 10^{-2}$
$\therefore pH=2-\log 4.95$
$=2-0.7=1.3$