Question

The pH of a 0.02M ammonia solution which is 5% ionized will be:
(A) 2
(B) 11
(C) 5
(D) 7

Answer 1

Hint: pH is a scale used to specify the acidity or basicity of an aqueous solution. Acidic solutions (solutions with higher concentrations of ${{H}^{+}}~$ ions) are measured to have lower pH values than basic or alkaline solutions.

Complete step by step answer:
The pH scale is logarithmic and inversely indicates the concentration of hydrogen ions in the solution. This is because the formula used to calculate pH approximates the negative of the base 10 logarithm of the molar concentration of hydrogen ions in the solution. More precisely, pH is the negative of the base 10 logarithm of the activity of the ${{H}^{+}}~$ ion.
The degree of dissociation of a weak electrolyte is proportional to the inverse square root of the concentration, or the square root of the dilution. The concentration of any one ionic species is given by the root of the product of the dissociation constant and the concentration of the electrolyte.
Given in the question is $\alpha $ = 5% = 0.05; degree of ionization,
\[N{{H}_{3}}+{{H}_{2}}O\to N{{H}_{4}}^{+}+O{{H}^{-}}\]

Concentration of ammonia given 0.02M, therefore, concentration of $O{{H}^{-}}$ will be:
\[\begin{align}
 & [O{{H}^{-}}] = 0.05\times 0.02 \\
 & [O{{H}^{-}}] = {{10}^{-3}}M \\
\end{align}\]

Since, pOH is $-log[O{{H}^{-}}]$,
Therefore, value of pOH will be,
\[\begin{align}
 & pOH=-\log {{10}^{-3}} \\
 & pOH = 3 \\
\end{align}\]

And relation of pH and pOH is given as: pH + pOH = 14
Therefore, pH=14-pOH
pH=14 - 3 = 11
So, the correct answer is “Option B”.

Note: The degree of Ionisation refers to the strength of an acid or a base. A strong acid is said to completely ionize in water whereas a weak acid is said to only ionise partially. According to the Arrhenius theory, the acids are the compounds that dissociate in the aqueous medium in order to generate the hydrogen ions, ${{H}^{+}}~$ in the aqueous medium.