The acid-dissociation constants of phosphoric acid (h3po4) are ka1 = 7.5 × 10-3, ka2 = 6.2 × 10-8, and ka3 = 4.2 × 10-13 at 25.0 °c. what is the molar concentration of phosphate ion in a 2.5 m aqueous solution of phosphoric acid?

Equation: [HA] = [H⁺] + [A⁻]

the concentration of the first dissociation:
[H⁺] =√7.5x10⁻³ x 2.5 = 0.1369

the concentration of the second dissociation:
[H⁺] =√6.2x10⁻⁸ x 0.1369 = 9.21x10⁻⁵

the concentration of the third dissociation:
[H⁺] =√4.2x10⁻¹³ x 9.21x10⁻⁵ = 6.22x10⁻⁹

we now them all : it adds up to [H⁺] = 0.3168

now find pH: pH = -log₁₀( 0.3168 ) = 0.5

hope that helps :)

The correct answer is d