Phosphoric acid ionizes in three steps with their ionization constant values Ka1, Ka2 and Ka3, respectively, while K is the overa…

Question

Phosphoric acid ionizes in three steps with their ionization constant values Ka1, Ka2 and Ka3, respectively, while K is the overall ionization constant.

Which of the following statements are true? (A) log K=log Ka1+log Ka2+log Ka3. (B) H3PO4 is a stronger acid than H2PO4^- and HPO4^2-. (C) Ka1Ka2Ka3. (D) Ka1=Ka3+Ka22.

1. A. A and B only
2. B. A and C only
3. C. B, C and D only
4. D. A, B and C only (Correct answer)

Correct Answer

Option D — A, B and C only

Detailed Solution & Explanation

The correct answer is A, B and C only.

Key Points

• The overall $K=K_{a_1}\cdot K_{a_2}\cdot K_{a_3}$, so $\log K=\log K_{a_1}+\log K_{a_2}+\log K_{a_3}$ — (A) true.
• Successive ionization constants fall sharply: $K_{a_1}=7.5\times10^{-3}>K_{a_2}=6.2\times10^{-8}>K_{a_3}=1.7\times10^{-12}$ — so (C) true and H$_3$PO$_4$ is the strongest acid, (B) true.
• (D) is a false arbitrary relation.