Phosphoric acid ionizes in three steps with their ionization constant values K a 1 , K a 2 and K a 3 , respectively, While K is the overall ionization constant. Which of the following statements are true ? A. log K =log K a 1 +log K a 2 +log K a 3 B. H 3 PO 4 is a stronger acid than H 2 PO 4 - and HPO 4 2- C. K a 1 K a 2 K a 3 D. K a 1 =frac K a 3 + K a 2 2 Choose the correct answer from the options given below:

Chemistry · Ionic Equilibrium

NEET (UG) 2025 — Question 55

Phosphoric acid ionizes in three steps with their ionization constant values $\mathrm{K}_{\mathrm{a}_{1}}, \mathrm{~K}_{\mathrm{a}_{2}}$ and $\mathrm{K}_{\mathrm{a}_{3}}$ , respectively,

While K is the overall ionization constant. Which of the following statements are true ?

A. $\log \mathrm{K}=\log \mathrm{K}_{\mathrm{a}_{1}}+\log \mathrm{K}_{\mathrm{a}_{2}}+\log \mathrm{K}_{\mathrm{a}_{3}}$

B. $\mathrm{H}_{3} \mathrm{PO}_{4}$ is a stronger acid than $\mathrm{H}_{2} \mathrm{PO}_{4}^{-}$ and $\mathrm{HPO}_{4}^{2-}$

C. $\mathrm{K}_{\mathrm{a}_{1}}>\mathrm{K}_{\mathrm{a}_{2}}>\mathrm{K}_{\mathrm{a}_{3}}$

D. $\mathrm{K}_{\mathrm{a}_{1}}=\frac{\mathrm{K}_{\mathrm{a}_{3}}+\mathrm{K}_{\mathrm{a}_{2}}}{2}$

Choose the correct answer from the options given below:

Option A:
A and B only
Option B:
A and C only
Option C:
B, C and D only
Option D:
A, B and C only
Correct

Answer: D

Step-by-step solution

$\mathrm{H}_{3} \mathrm{PO}_{4} \rightleftharpoons \mathrm{H}^{+}+\mathrm{H}_{2} \mathrm{PO}_{4}^{-}$ ; $\mathrm{Ka}_{1}$

$\mathrm{H}_{2} \mathrm{PO}_{4}^{-} \rightleftharpoons \mathrm{H}^{+}+\mathrm{HPO}_{4}^{2-} ; \mathrm{Ka}_{2}$

$\mathrm{HPO}_{4}^{2-} \rightleftharpoons \mathrm{H}^{+}+\mathrm{PO}_{4}^{3-} ; \mathrm{Ka}_{3}$

$\mathrm{H}_{3} \mathrm{PO}_{4} \rightleftharpoons 3 \mathrm{H}^{+}+\mathrm{PO}_{4}^{3-} ; \mathrm{K}=\mathrm{Ka}_{1} \times \mathrm{Ka}_{2} \times \mathrm{Ka}_{3}$

For polybasic acid, order of ionization constant is $\mathrm{Ka}_{1}>\mathrm{Ka}_{2}>\mathrm{Ka}_{3}$ because as negative charge increases,

tendency to loose $\mathrm{H}^{+}$ ions decreases.

Overall $\mathrm{K}=\mathrm{Ka}_{1} \times \mathrm{Ka}_{2} \times \mathrm{Ka}_{3}$

Taking log on both sides

$\log \mathrm{K}=\log \mathrm{Ka}_{1}+\log \mathrm{Ka}_{2}+\log \mathrm{Ka}_{3}$

Answer key and solution verified before publishing.

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Exam: NEET (UG) 2025
Paper: 2025 paper
Subject: Chemistry
Chapter: Ionic Equilibrium
Topic: Fundamental Definitions