Which of the following is the weakest base?

The correct option is C

${\mathrm{NH}}_4\mathrm{OH}$

The explanation for the correct option (C):

• Weak bases are those bases that ionize partially in Water (H₂O) to produce Hydroxide ions (OH^-).
• Examples: Ammonia$\left({\mathrm{NH}}_3\right)$ and Zinc hydroxide ($\mathrm{Zn}{\left(\mathrm{OH}\right)}_2$)
• $\mathrm{Zn}{\left(\mathrm{OH}\right)}_2\left(\mathrm{aq}\right)\rightleftharpoons {\mathrm{Zn}}^{2+}\left(\mathrm{aq}\right)+2{\mathrm{OH}}^{-}\left(\mathrm{aq}\right)$
• Since, Ammonium hydroxide (${\mathrm{NH}}_4\mathrm{OH}$) does not ionize completely to produce Hydroxide ions when dissolved in Water, it acts as a weak base.
• ${\mathrm{NH}}_4\mathrm{OH}\left(\mathrm{aq}\right)\rightleftharpoons {\mathrm{NH}}_4^{+}\left(\mathrm{aq}\right)+{\mathrm{OH}}^{-}\left(\mathrm{aq}\right)$

The explanation for the incorrect options:

(A)

• When Sodium hydroxide ($\mathrm{NaOH}$) is dissolved in Water, it ionizes completely to produce Hydroxide ions. Hence, it acts as a strong base.
• $\mathrm{NaOH}\left(\mathrm{aq}\right)\to {\mathrm{Na}}^{+}\left(\mathrm{aq}\right)+{\mathrm{OH}}^{-}\left(\mathrm{aq}\right)$

(B)

• When Calcium hydroxide ($\mathrm{Ca}{\left(\mathrm{OH}\right)}_2$) is dissolved in Water, it ionizes completely to produce Hydroxide ions. Hence, it acts as a strong base.
• $\mathrm{Ca}{\left(\mathrm{OH}\right)}_2\left(\mathrm{aq}\right)\to {\mathrm{Ca}}^{2+}\left(\mathrm{aq}\right)+2{\mathrm{OH}}^{-}\left(\mathrm{aq}\right)$

(D)

• When Potassium hydroxide ($\mathrm{KOH}$) is dissolved in Water, it ionizes completely to produce Hydroxide ions. Hence, it acts as a strong base.
• $\mathrm{KOH}\left(\mathrm{aq}\right)\to {\mathrm{K}}^{+}\left(\mathrm{aq}\right)+{\mathrm{OH}}^{-}\left(\mathrm{aq}\right)$

Final Answer: Hence, option (C) is correct. ${\mathrm{NH}}_4\mathrm{OH}$ is the weakest base.