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
• D. 11.3

Answer 1

Answer: (D)

11.3

Answer 2

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 $=14-2.70$
$=11.3$