Question

Phosphoric acid (h3po4) has three acid dissociation constants ( ka). the first dissociation constant has the greatest value, and it applies to the reaction described by which equation?

Answer 1

Missing question:
A. H3PO4 (aq)<--> H+ (aq) + H2PO4- (aq).
B. H2PO4- (aq)<--> H+ (aq) + H2PO4-^2- (aq).
C. H3PO4 (aq)<--> 2H+ (aq) + HPO4^2-(aq).
D. HPO4^2- (aq)<--> H+ (aq) + PO4^3- (aq).
Answer is: A. H3PO4 (aq)<--> H+ (aq) + H2PO4- (aq).
The first dissociation constant :Ka₁ = [H+]·[H₂PO₄⁻] / [H₃PO₄].
In first dissociation phosphoric acid lost one proton (hydrogen ion).

Answer 2

Ans: H₃PO₄ ↔ H⁺ + H₂PO₄⁻

Phosphoric acid is a weak triprotic acid. The three acidic protons are released  in a stepwise manner and are represented in terms of their dissociation constants, ka values.

Step 1: H₃PO₄ ↔ H⁺ + H₂PO₄⁻         Ka₁ = 7.1 *10⁻³

Step 2: H₂PO₄⁻ ↔ H⁺ + HPO₄²⁻       Ka₂ = 6.3 *10⁻⁸

Step 3: HPO₄²⁻ ↔ H⁺ + PO₄³⁻         Ka₃ = 4.5 *10⁻¹³

Since Ka1 is the greatest, the reaction is described by step 1.