Question

When 200 mL of aqueous solution of $HCl$ (pH = 2) is mixed with 300 mL of an aqueous solution of NaOH (pH = 12), the pH of the resulting mixture is

• A. 10
• B. 2.7
• C. 4.0
• D. 11.3
• E. 2

Answer 1

Answer: (D)

Concentration of $HCl$ solution $=1\times {10}^{-2}$
$\therefore$ Millimoles of $HCl$ solution $=200\times 1\times {10}^{-2}$
$=2$ Similarly, millimoles of $NaOH$ solution $=300\times 1\times {10}^{-2}$
$=3$ Concentration of the resultant solution
$=\frac{3-2}{300+200}$ $=\frac{1}{500}=0.2\times {10}^{-2}$
$\therefore$ $[O{H}^{-}]=0.2\times {10}^{-2}$
$pOH=-\log [O{H}^{-}]=-\log [2\times {10}^{-3}]=+2.70$
$\therefore$ pH of the resulting mixture $=14-2.70$
$=11.3$