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 (\( H_3PO_4 \)) undergoes stepwise ionization: 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 the product of the individual constants: \[ K = K_1 \times K_2 \times K_3 \] Taking the logarithm of both sides yields: \[ \log K = \log K_1 + \log K_2 + \log K_3 \] Statement A is true. - Statement B: \( H_3PO_4 \) is a stronger acid than \( H_2PO_4^- \) and \( HPO_4^{2-} \) because it is fully protonated. Each successive deprotonation weakens the acid. Statement B is true. - Statement C: The ionization constants follow the order \( K_1>K_2>K_3 \). The first proton is most easily removed, with subsequent deprotonations becoming progressively more difficult. Statement C is true. - Statement D: This is incorrect. The ratio \( \frac{K_3}{K_2} \) has no standard relationship concerning phosphoric acid ionization. Statement D is false. The correct statements are A, B, and C.