Question

$N{H_4}OH$ is a weak base because:
A.It is completely ionised
B.It has a low vapour density
C.It has a low density
D.It is only partially ionised

Answer 1

To answer this question, you should recall the concept of ionisation of weak bases. Any base which is called a weak base is dependent on the value of ionization constant ${\text{(}}{{\text{K}}_{\text{b}}}{\text{)}}$. ${{\text{K}}_{\text{b}}}$ of any base helps determine how completely a base dissociates into its component ions in water.

Complete step by step answer:
We know that $N{H_4}OH$ dissociates in water as $N{H_3} + {H_2}O \to N{H_4}^ + + O{H^ - }$. This reaction has ${{\text{K}}_{\text{b}}} = 1.8 \times {10^{ - 5}}$. You can notice that the value of ${{\text{K}}_{\text{b}}}$ is quite small, meaning that very little ammonium and hydroxide forms in the solution.
In other words, the concentration of $O{H^ - }$ is much smaller than the nominal ammonia or “ammonium hydroxide” concentration. That’s why it is a weak base.
Another point to be noted is that the relative sizes of $N{H_4}^ +$ and hydroxide ions are comparable. This leads to a strong bond between ammonium ion and Hydroxide ion. As a result, when dissolved in water, $N{H_4}OH$ doesn't dissociate completely leading to its weak basic character. Hence, $N{H_4}OH$ is a weak base because it partially dissociates into $N{H_4}^ +$and $O{H^ - }$ ions in aqueous solutions and the amount of $O{H^ - }$ ions produced is low as it is partially ionised.
Therefore, we can conclude that the correct answer to this question is option D.

Note: You should remember the difference between strong bases and weak bases: a strong base is a base that is $100\%$ ionized in solution but if it is less than $100\%$ ionized in solution, it is a weak base. There are very few strong bases and certain salts will also affect the acidity or basicity of aqueous solutions because some of the ions will undergo hydrolysis. The general rule is that salts with ions that are part of strong acids or bases will not hydrolyse, while salts with ions that are part of weak acids or bases will hydrolyse.