Phosphoric acid ionizes in three steps with their ionization constant values K_a1,K_a2,K_a3, respectively, while K is the overall ionization constant. Which of the following statements are true?

A. log K = log K_a1 + logK_a2 + logK_a3

B. H₃PO₄ is a stronger acid than  H₂PO₄^- and HPO^2-₄

C. K_a1 > K_a2 > K_a3

D.  K_a1 = K_a3 + K_a2/2

Choose the correct answer from the options given below :

**Statement A:** The overall ionization constant K for phosphoric acid is the product of the three stepwise constants:

$K\; =\; K$_a1·K_a2·K_a3

Taking the base‑10 logarithm of both sides gives

$log K\; =\; log K$_a1 + log K_a2 + log K_a3

Hence statement A is true.

**Statement B:** The first dissociation of phosphoric acid is

$H₃PO₄\; \rightleftharpoons \; H₂PO₄⁻\; +\; H⁺\hspace{1em}\hspace{1em}K$_a1

Since K_a1 (≈ 7.5 × 10⁻³) is larger than K_a2 and K_a3, the parent acid H₃PO₄ donates a proton more readily than its conjugate bases H₂PO₄⁻ and HPO₄²⁻. Therefore H₃PO₄ is the strongest acid among the three species, making statement B true.

**Statement C:** For a polyprotic acid each successive deprotonation is less favorable because the negative charge on the conjugate base builds up, repelling the departing proton. Consequently

$K$_a1 > K_a2 > K_a3

This ordering is observed for phosphoric acid (K_a1 ≈ 7.5 × 10⁻³, K_a2 ≈ 6.2 × 10⁻⁸, K_a3 ≈ 4.8 × 10⁻¹³). Thus statement C is true.

**Statement D:** No theoretical or experimental relationship of the form

$K$_a1 = K_a3 + K_a2/2

exists for phosphoric acid. Substituting the actual values gives a clear mismatch (7.5 × 10⁻³ ≠ 4.8 × 10⁻¹³ + ½·6.2 × 10⁻⁸). Therefore statement D is false.

Since statements A, B, C are true and D is false, the correct option is **A, B and C only**.