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 $