a) A polyfunctional acid is an acid that has more than one functional group.
b) The equations of dissociation of phosphoric acid are:
H₃PO₄ + H₂O ⇄ H₂PO₄⁻ + H₃O⁺ H₂PO₄⁻ + H₂O ⇄ HPO₄²⁻ + H₃O⁺ HPO₄²⁻ + H₂O ⇄ PO₄³⁻ + H₃O⁺
c) The equation for calculating the concentration of H₃O⁺ is
a) A polyfunctional acid can be defined as an acid that has more than one functional group. Phosphoric acid (H₃PO₄) is an example of polyfunctional acid since it is composed of three hydroxyl groups joined to a phosphorus atom, which is also joined to an oxygen atom by a double bound. In that structure, the three hydrogen atoms of the hydroxyl groups give the acidic behavior to this compound.
b) Phosphoric acid has three equations of dissociation:
H₃PO₄ + H₂O ⇄ H₂PO₄⁻ + H₃O⁺ (1)H₂PO₄⁻ + H₂O ⇄ HPO₄²⁻ + H₃O⁺ (2)HPO₄²⁻ + H₂O ⇄ PO₄³⁻ + H₃O⁺ (3)
The dissociation constants for the three above equations are:
(4)
(5)
(6)
c) We can calculate the concentration of H₃O⁺ for each equilibrium with the equations (4), (5), and (6).
The general reaction of dissociation of phosphoric acid is given by the sum of equations (1), (2), and (3):
H₃PO₄ + 3H₂O ⇄ PO₄³⁻ + 3H₃O⁺ (7)
The concentration of H₃O⁺ for the total dissociation reaction (eq 7) can be found as follows:
(8)
Where:
Hence, by knowing the dissociation constants K₁, K₂ and K₃, and the concentrations of PO₄³⁻ and H₃PO₄, the [H₃O⁺] is:
You can find more about acid dissociation constant in the link:
I hope it helps you!