Question

Which of the following is a weak base?
A) $\text{ N}{{\text{H}}_{\text{4}}}\text{OH}$
B) $\text{N}{{\text{H}}_{\text{3}}}$
C) $\text{KOH}$
D) Both A and B

Answer 1

Hint: The strong electrolyte completely dissociates into the solution however the weak electrolytes partially dissociate in the solution. We can say that the base which easily donates the hydroxide ion is weak. Ammonium hydroxide and ammonia do not completely dissociate in the solution but potassium hydroxide can go complete ionization.


Complete step by step answer:
According to Lowry-Bronsted concept of acid and base, acid is defined as the substance which tends to donate a proton to any other substance and base is a substance that tends to accept a proton from any other substance. That is, acid is a proton donor and base is a proton acceptor.
Ammonia is a weak base. Now according to the Lowry and Bronsted theory the ammonia can accept the proton from the acid. Ammonia $\text{N}{{\text{H}}_{\text{3}}}$ because the lone pair can accept the hydrogen ion. It reacts with the water and abstracts a proton from the water to produce the ammonium ion $\text{(NH}_{\text{4}}^{\text{+}}\text{)}$ and hydroxide ion$(\text{O}{{\text{H}}^{-}})$.
The reaction is reversible and at any of the time $99{\scriptstyle{}^{0}/{}_{0}}$ of the ammonia is still present in its original state (undergoes no reaction) but only $1{\scriptstyle{}^{0}/{}_{0}}$ of the ammonia molecules produce the hydroxide ion. the reaction is as depicted below,
$\text{ N}{{\text{H}}_{\text{3}}}\text{(aq) + }{{\text{H}}_{\text{2}}}\text{O(l) }\rightleftharpoons \text{ NH}_{\text{4}}^{\text{+}}\text{(aq) + O}{{\text{H}}^{-}}\text{(aq)}$
Ammonia can react with acid to form ammonium salt. But as it does not undergo complete ionization it is a weak base.
Similarly, the ammonium hydroxide $\text{ N}{{\text{H}}_{\text{4}}}\text{OH}$ undergoes incomplete dissociation or partial dissociation. the equilibrium of the ammonium hydroxide is as shown below,
$\text{ N}{{\text{H}}_{\text{4}}}\text{OH}\rightleftharpoons \text{ NH}_{4}^{+}+\text{O}{{\text{H}}^{-}}$
The ammonium hydroxide does not undergo $100{\scriptstyle{}^{0}/{}_{0}}$ dissociation. Therefore, ammonium hydroxide is also a weak base.
Now, $\text{KOH}$ has the strong ability to donate the hydroxide ion$(\text{O}{{\text{H}}^{-}})$. It undergoes complete ionization to generate$\text{ }{{\text{K}}^{\text{+}}}$ and $(\text{O}{{\text{H}}^{-}})$ ion in solution. The reaction is as depicted below,
$\text{KOH }\to {{\text{K}}^{\text{+}}}\text{ + O}{{\text{H}}^{-}}$
Therefore, it is a strong base.
Thus, $\text{N}{{\text{H}}_{\text{3}}}$ and $\text{ N}{{\text{H}}_{\text{4}}}\text{OH}$ are the weak base.

Hence, D) is the correct option.


<>Note: Generally we see that the bases can donate the hydroxide ion. But ammonia does not contain the hydroxide ion but it can react with the water to generate ammonium ion. The basicity may depend on the various concepts. Such as in the Lewis acid-base concept, the electron donors are base, and the acceptor is called as the acid.eg. Ammonia and $\text{B}{{\text{F}}_{\text{3}}}$