Question

What will be the resultant pH when 200 mL of an aqueous solution of HCl (pH = 2.0) is mixed with 300 mL of an aqueous solution of NaOH (pH = 12.0)

[Note : write the value to the nearest integer].

Answer 1

pH 2.0 of HCl corresponds to the hydrogen ion concentration of 0.01 M.
The number of moles of hydrogen ions present are $0.01\times 0.2=0.0020$

pH 12.0 of NaOH corresponds to hydroxide ion concentration of 0.01 M.
The number of moles of hydroxide ions present are $0.01\times 0.3=0.0030$

Out of 0.0030 moles of hydroxide ions, 0.0020 moles will be neutralized with 0.0020 moles of hydrogen ions.

Hence, the hydroxide ion concentration remaining is $\frac{0.0010}{0.5}=0.0020M$

Hence, the pOH of the solution is $pOH=-\log \left[O{H}^{-}\right]=-\log 0.0020=2.699$

The pH of the solution is $pH=14-pOH=14-2.699=11.301\approx 11$.