Question

A weak mono acidic base is 5% ionized in 0.01M solution. The hydroxide ion concentration in the solution is:
A. $5 \times {10^{ - 2}}$
B. $5 \times {10^{ - 4}}$
C. $5 \times {10^{ - 10}}$
D. $2 \times {10^{ - 11}}$

Answer 1

Hint: Weak Monoacidic base only give one hydroxide ion like $N{H_4}OH$ and here concentration is given, degree of dissociation is given then we can calculate concentration of hydroxide ion.Weak acid does not dissociate 100% completely .

Complete solution:
When one molecule of a base via complete ionization produces one hydroxide ion, the base is said to be a monoacidic base. Examples of monoacidic bases are: Sodium hydroxide, potassium hydroxide, silver hydroxide, ammonium hydroxide, etc.
Suppose we have $N{H_4}OH$ as a weak monoacidic base
 Let concentration of $N{H_4}OH$ be C and degree of dissociation be $\alpha $
               $N{H_4}OH \rightleftharpoons N{H^{4 + }} + O{H^ - }$
$ t=0 \;\;\;\;\;\;\;\;\;\; C \;\;\;\;\;\;\;\;\;\;\;\; 0 \;\;\;\;\;\;\;\;\;\;\;\; 0 \\
   t=t \;\;\;\;\;C-C\alpha\;\;\;\;\;\;\;C\alpha \;\;\;\;\;\;\;\;C\alpha $
Here we can see concentration of =C$\alpha $
 $
 = 0.01 \times \dfrac{5}{{100}} \\
 = 5 \times {10^{ - 4}}
$
So concentration of hydroxide ion is $ = 5 \times {10^{ - 4}}$

Correct Option: B. $5 \times {10^{ - 4}}$

Additional Information:
1. Monoacidic base: Monoacidic base produces only one hydroxyl ion ($OH^-$) per molecule in water. Example: Sodium hydroxide, Ammonium hydroxide.
2. Diacidic base: Diacidic base produces two hydroxyl ions ($OH^-$) per molecule in water. Example:Calcium hydroxide, copper (II) hydroxide.
3. Triacidic base: Triacidic base produces three two hydroxyl ions ($OH^-$) per molecule in water. Example: Aluminium hydroxide, ferric hydroxide

Note:
Remember the definition of monoacidic base and monobasic acid to solve these type of questions.Weak acids do not dissociate completely so will need to take help of degree of dissociation as we have done in this question.