Question

Phosphoric acid ionizes in three steps with their ionization constant values $ K_1 $, $ K_2 $, and $ K_3 $, respectively, while $ K $ is the overall ionization constant.
Which of the following statements are true?

• A. A and C only
• B. B, C and D only
• C. A, B and C only
• D. A and B only

Hint:

For polyprotic acids like phosphoric acid, the ionization constants decrease as we move from the first proton dissociation to the third, reflecting the decreasing strength of successive deprotonations.

Answer 1

The Correct Option is C

Phosphoric acid (\( H_3PO_4 \)) ionizes stepwise as: 1. \( H_3PO_4 \xrightarrow{K_1} H_2PO_4^- + H^+ \) 2. \( H_2PO_4^- \xrightarrow{K_2} HPO_4^{2-} + H^+ \) 3. \( HPO_4^{2-} \xrightarrow{K_3} PO_4^{3-} + H^+ \) The overall ionization constant \( K \) is related to the individual constants by the equation: \[ K = K_1 \times K_2 \times K_3 \] Taking the logarithm of both sides: \[ \log K = \log K_1 + \log K_2 + \log K_3 \]
Thus, Statement A is true.
- Statement B: \( H_3PO_4 \) (phosphoric acid) is indeed a stronger acid than \( H_2PO_4^- \) (dihydrogen phosphate) and \( HPO_4^{2-} \) (hydrogen phosphate) because it is in its protonated form, and each successive deprotonation reduces the acid strength.
Thus, Statement B is true.
- Statement C: As for the ionization constants, \( K_1>K_2>K_3 \), because the first proton is the easiest to lose, and subsequent deprotonations are progressively harder.
Therefore, Statement C is true.
- Statement D: This is incorrect. The ratio \( \frac{K_3}{K_2} \) does not have any standard relationship in the context of phosphoric acid's ionization.
Therefore, Statement D is false.

Thus, the correct answer is A, B, and C only.