Question

Which of the following is a weak base?
(A) ${\text{NaOH}}$
(B) ${\text{KOH}}$
(C) ${\text{N}}{{\text{H}}_{\text{4}}}{\text{OH}}$
(D) ${\text{Ca}}{\left( {{\text{OH}}} \right)_{\text{2}}}$

Answer 1

Hint: Strong bases are those bases which completely ionizes in water and produce a large amount of ${\text{O}}{{\text{H}}^ - }$ ions and weak bases are those bases which partially ionizes in water to produce ${\text{O}}{{\text{H}}^ - }$ ions.

Complete step by step answer:
> ${\text{NaOH}}$ is classified as a strong base because it is associate completely in aqua solution to form sodium cations ${\text{N}}{{\text{a}}^{\text{ + }}}$ and hydroxide anions ${\text{O}}{{\text{H}}^ - }$.
> ${\text{KOH}}$ or potassium hydroxide is composed of the hydroxide anions ${\text{O}}{{\text{H}}^ - }$, which makes it a strong base. In solution the hydroxide anion will completely react with any available protons that is why ${\text{KOH}}$ is a strong base. It is not an acid of any type, weak or strong since it does not contribute any protons to solution.
${\text{N}}{{\text{H}}_{\text{4}}}{\text{OH}}$ is a weak base because it partially dissociates into ${\text{N}}{{\text{H}}^{\text{ + }}}$ and ${\text{O}}{{\text{H}}^ - }$ ions in aqueous solutions and the amount of ${\text{O}}{{\text{H}}^ - }$ ions produced is low.
> Calcium hydroxide or ${\text{Ca}}{\left( {{\text{OH}}} \right)_{\text{2}}}$is a strong base. It dissociates completely into ${\text{C}}{{\text{a}}^{{\text{2 + }}}}$and ${\text{O}}{{\text{H}}^ - }$ ions in aqueous solution. It is however sparingly soluble in water. ${\text{Ca}}{\left( {{\text{OH}}} \right)_{\text{2}}}$is a strong base but not very soluble.
Therefore, the correct answer to the question is Option C.

Note: Here are the pH values of the mentioned options:
- The pH value of ${\text{NaOH}}$ is 10.98.
- The pH value of ${\text{KOH}}$ is 11.95.
- The pH value of ${\text{N}}{{\text{H}}_{\text{4}}}{\text{OH}}$ is 10.09.
- The pH value of ${\text{Ca}}{\left( {{\text{OH}}} \right)_{\text{2}}}$is 12.20.
From the pH values we can see that the weakest base is ${\text{N}}{{\text{H}}_{\text{4}}}{\text{OH}}$.