Question

The ${K_a}$ values for $HP{O_4}^{2 - }$ and $HS{O_3}^ - $ are $4.8 \times {10^{ - 13}}$ and $6.3 \times {10^{ - 8}}$ respectively. Therefore, it follows that $HP{O_4}^{2 - }$ is a _____________ acid than $HS{O_3}^ - $ and $P{O_4}^{3 - }$ is a _____________ base than $S{O_3}^{2 - }.$
A. weaker, stronger respectively
B. stronger, weaker respectively
C. weaker, weaker respectively
D. stronger, stronger respectively

Answer 1

Hint: While solving such types of fill in the blanks question we need to keep in mind the concept of dissociation constant ${K_a}$ for an acid. Here, we need to refer to the Arrhenius theory related to acid and base. Then, we can easily determine any strong acid or and weak acid or base.

Complete step by step answer:
From the concepts of acid or base dissociation constant we know that the greater the value of the dissociation constant the greater is the strength of an acid. It implies that the strength of an acid is directly proportional to the value of the dissociation constant ${K_a}$
Acid or base dissociation constant
${K_a} = \dfrac{{[{A^ + }][{B^ - }]}}{{[AB]}}$
As acid dissociation constant is the measure of the strength of an acid. So, with the help of Arrhenius theory we will determine that $HP{O_4}^{2 - }$ is a weaker acid than $HS{O_3}^ - $as $4.8 \times {10^{ - 13}}$ is a lesser value in comparison to$6.3 \times {10^{ - 8}}$.
Now, we need to refer to the concept of a conjugate base to solve the other half of the question. Conjugate base is the ${B^ - }$ part of a base. In case of a conjugate base of a strong acid is weak and conjugate base of a weak acid is strong.
Thus, $P{O_4}^{3 - }$ is a stronger base than $S{O_3}^{2 - }$
Therefore, we can mark the correct option as (A).

Note:
-While solving this question, we need to remember that a conjugate acid is one that is formed after the base accepts a proton whereas a conjugate base is one that is formed after the acid releases its proton.
-Acid dissociation constant is the equilibrium constant that refers to the ionization or dissociation of an acid.