NEET 2025 — Chemistry solution

NEET2025Chemistry

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. $lo g K = lo g K_{a_{1}} + lo g K_{a_{2}} + lo g K_{a_{3}}$

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:

A
A and B only
B
A and C only
C
B, C and D only
D
A, B and C only

Solution

H $_{3}$ PO $_{4} ⇌$ H $^{+}$ + H $_{2}$ PO $_{4}^{-} (K_{a_{1}})$

H $_{2}$ PO $_{4}^{-} ⇌$ H $^{+}$ + HPO $_{4}^{2 -} (K_{a_{2}})$

HPO $_{4}^{2 -} ⇌$ H $^{+}$ + PO $_{4}^{3 -} (K_{a_{3}})$

Overall: $K = K_{a_{1}} \cdot K_{a_{2}} \cdot K_{a_{3}}$ , hence $lo g K = lo g K_{a_{1}} + lo g K_{a_{2}} + lo g K_{a_{3}}$ . A is true.

Removing successive protons becomes harder as the species gets more negatively charged, so $K_{a_{1}} > K_{a_{2}} > K_{a_{3}}$ ; equivalently, H $_{3}$ PO $_{4}$ is a stronger acid than H $_{2}$ PO $_{4}^{-}$ , which is stronger than HPO $_{4}^{2 -}$ . B and C are true.

D is just an arbitrary algebraic relation with no physical basis — false.

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