Question

What is the equation of acid ionization of $HN{O_3}$?
A. ${K_a} = \dfrac{{\left[ {{H^ + }} \right]\left[ {NO_3^ - } \right]}}{{\left[ {HN{O_3}} \right]}}$
B. ${K_a} = \dfrac{{\left[ {HN{O_3}} \right]}}{{\left[ {{H^ + }} \right]\left[ {NO_3^ - } \right]}}$
C. ${K_a} = \left[ {{H^ + }} \right]\left[ {NO_3^ - } \right]$
D. None of these

Answer 1

We need to remember that the acid can turn blue litmus red and reacts with base to form an aqueous solution whereas base turns red litmus blue and similarly reacts with acid to produce an aqueous solution. We also remember that $HN{O_3}$ is nitric acid which is generally a weak acid.

Complete step by step answer:
Before solving this question let’s do a comparative study on acid and base.
- Acid is an ion or molecule that can donate a proton ${H^ + }$ whereas a base is a molecule or ion that can accept a proton ${H^ + }$.
- Acids have a pH lower than $7$ whereas a base has a pH higher than $7$.
Ionization of weak acid HA, can be written as
$HA \to {H^ + } + {A^ - }$
Acid ionization constant is the equilibrium constant for ionization of an acid which can be represented as:
${K_a} = \dfrac{{\left[ {{H^ + }} \right]\left[ {{A^ - }} \right]}}{{\left[ {HA} \right]}}$
Acid ionization tells us how much acid is being ionized in a solution.
Acid ionization of $HN{O_3}$ can be represented as:
$HN{O_3} \to {H^ + } + N{O_3}^ -$
${K_a} = \dfrac{{\left[ {{H^ + }} \right]\left[ {NO_3^ - } \right]}}{{\left[ {HN{O_3}} \right]}}$
Option A) this is a correct option as explained above $HN{O_3} \to {H^ + } + N{O_3}^ -$ nitric acid dissociates itself into ${H^ + }$ and $N{O_3}^ -$ ions and ionization constant is represented as dissociation of ions divided by the concentration of $HN{O_3}$.
Option B) This is an incorrect option as explained above.
Option C) this is an incorrect option as it does not represent acid ionization constant.
Option D) this is an incorrect option as we got a correct answer.

Hence, the correct answer is, ‘Option A’.

Note: We have to remember that the conjugate acid is a species that is formed when an acid donates a${H^ + }$ to a base or you can refer to it as a base on which a proton is attached.
We also know that the conjugate base can be referred to as the remaining part of a conjugate acid or you can say that species that accept a ${H^ + }$.