Phosphoric acid ionizes in three steps with their ionization constant values , and respectively, while is the overall ionization constant. Which of the following statements are true? [2025]

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Solution

Q.
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: [2025]

1 B, C and D only

2 A, B and C only

3 A and B only

4 A and C only

Ans.
(2)

$K = K_{a_{1}} \times K_{a_{2}} \times K_{a_{3}}$

Hence, $\log K = \log K_{a_{1}} + \log K_{a_{2}} + \log K_{a_{3}}$

It is difficult to remove a proton from negatively charged ion, hence, $H_{3} {PO}_{4}$ is stronger acid than $H_{2} {PO}_{4}^{-}$ and ${HPO}_{4}^{2 -}$ .

Higher order ionisation constants $(K_{a_{2}}, K_{a_{3}})$ are smaller than the lower order ionisation constant $(K_{a_{1}})$ . Hence, $K_{a_{1}} > K_{a_{2}} > K_{a_{3}}$ .

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