Question

$N{H_4}CN$ is a salt of weak acid HCN $({K_a} = 6.2 \times {10^{ - 10}})$ and a weak base $N{H_4}OH$ $({K_b} = 1.8 \times {10^{ - 5}})$. A one molar solution of $N{H_4}CN$ will be?
(A) Neutral
(B) Strongly acidic
(C) Strongly basic
(D) Weakly basic

Answer 1

We will need to recall the concept of $p{K_a}$ in order to answer this question. So $p{K_a}$ value is used to indicate the strength of an acid. $p{K_b}$ indicates the strength of a base.

Complete step by step answer:
Certain reactions are extremely slow and some extremely fast that’s the reason why their progress can not be determined by any factor. Those reactions that are moderately fast can be observed and their speed can be calculated. These calculations are done in terms of rate of reaction ‘$p{K_a}$’ which is called the dissociation constant.
- Dissociation constant of any acid or base is the ratio of the concentration of resultant ions to the concentration of the undissociated acid or base.
- $p{K_a}$ can be obtained by the negative logarithmic value on the base of 10 of the dissociation constant of the acid.
As we already know $p{K_a} = - \log {K_a}$
So, as we have already seen, the smaller the $p{K_a}$, the stronger the acid. Generally very strong acids have $p{K_a}$ less than 1 and moderately strong acids have $p{K_a}$ in between 1 to 5 and weak acids have $p{K_a}$ above 5.

- Let’s see how ammonium cyanide will dissociate.
$N{H_4}CN \to N{H_4}^ + + C{N^ - }$
So, we can say that ${K_a}$ is directly proportional to the ${H^ + }$(proton) concentration and ${K_b}$ is directly proportional to the $O{H^ - }$ (hydroxide ion) concentration.
Therefore, in the given question ${K_b} > {K_a}$ and ${K_b} < < 1$. Thus we can predict that the solution of $N{H_4}CN$ is weakly basic.
So, the correct answer is “Option D”.

Note: Do not get confused between pH and $p{K_a}$. The main difference between them is pH tells the concentration of hydrogen ions in a certain medium while $p{K_a}$ value indicates the degree at which an acid dissociates.