Question

Write the three dissociation equations for phosphoric acid and write the Ka expressions.

Answer 1

Phosphoric acid dissociates in three steps each with its own Ka expression indicating the strength of the acid, with each step's ionization becoming progressively weaker.

Step-by-step explanation:

Phosphoric acid is a triprotic acid, meaning it can donate three protons (H+ ions), and it dissociates in a stepwise manner. Each dissociation has an associated equilibrium constant, denoted as Ka1, Ka2, and Ka3 for the first, second, and third dissociation steps, respectively.

Dissociation Equations and Ka Expressions

1. H3PO4 (aq) => H+ (aq) + H2PO4- (aq)
   Ka1 = [H+][H2PO4-]/[H3PO4]
2. H2PO4- (aq) => H+ (aq) + HPO42- (aq)
   Ka2 = [H+][HPO42-]/[H2PO4-]
3. HPO42- (aq) => H+ (aq) + PO43- (aq)
   Ka3 = [H+][PO43-]/[HPO42-]

The ionization constants indicate the strength of the acid, with successive Ka values becoming smaller as the degree of ionization in each step is significantly weaker, a characteristic of polyprotic acids.