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? Choose the correct answer from the options given below :
- B, C and D only
- A, B and C only
- A and B only
- A and C only
Answer
(2) A, B and C only
Solution
Overall ionisation: H3PO4 3H+ + PO43-
K = Ka1 Ka2 Ka3 (overall constant = product of the stepwise constants).
A. Taking logarithms: K = Ka1 + Ka2 + Ka3. True.
B. As per NCERT, it is easier to remove H+ from a neutral molecule than from a negatively charged ion, so H3PO4 is a stronger acid than H2PO4- and HPO42-. True.
C. For the same reason Ka1 > Ka2 > Ka3. True.
D. The stepwise constants are not related as Ka1 = Ka3 + Ka22; in fact Ka1 is much larger than both Ka2 and Ka3. False.
A, B and C only.
Answer: (2)