Question

What will be the resultant $pH$ when $200mL$ of an aqueous solution of $HCl$ $(pH = 2.0)$ is mixed with $300mL$ of an aqueous solution of $NaOH$ $(pH = 12.0)$.
[Note :write the value to the nearest integer].

Answer 1

Hint: At first think about the $pH$of an aqueous solution. $pH$ is defined as a scale used to specify the acidity or basicity of an aqueous solution.

Complete step by step answer:
$pH$ is really a measure of the relative amount of free hydrogen and hydroxyl ions in the water. $pH$ is given by $ - \log [{H^ + }]$.
The hydrogen ion concentration of hydrochloric acid of $pH = 2.0$ is $0.01M$.
The number of moles of hydrogen ions present are $0.01 \times 0.2 = 0.0020$
The hydrogen ion concentration of sodium hydroxide of $pH$ $ = 12.0$ is $0.01M$.
The number of moles of hydrogen ions present are $0.01 \times 0.3 = 0.0030$
In $0.0030$ moles of hydroxide ions, $0.0020$ moles will be neutralized with $0.0020$ moles of hydrogen ions.
The remaining hydroxide ion concentration is $\dfrac{{0.0010}}{{0.5}} = 0.0020M$
We all know that, $pOH = - \log [O{H^ - }]$
The $pOH$ of the solution is $pOH = - \log 0.0020 = 2.699$
The $pH$ of the solution is
  $
  pH = 14 - pOH \\
  pH = 14 - 2.699 \\
  pH = 11.301 \\
  pH \approx 11 \\
 $
The resultant $pH$ of the solution is $11$.

Note:
The $pH$ scale is logarithmic and inversely indicates the concentration of hydrogen ions in the solution. Lower $pH$ values correspond to solutions which are more acidic in nature whereas higher values correspond to solutions which are more basic or alkaline.