Answer to Question #106560 in Physical Chemistry for Angelie Ausmolo

Question

Compute for the [PO4 ^3-] of a 0.025M H3PO4 solution

Answer 1

The acidity constants of H₃PO₄ are the following:

1) H₃PO₄ = H⁺ + H₂PO₄^- Ka1 = 7.25*10^-3

2) H₂PO₄^- = H⁺ + HPO₄^2- Ka2 = 6.31*10^-8

3) HPO₄^2- = H⁺ + PO₄^3- Ka3 = 3.98*10^-13

The first equilibrium is predominant, as its constant is the highest. Hence, the concentration of H⁺ in solution is determined by reaction 1). Since H₂PO₄^- is produced in the same quantity as H⁺, their concentrations can be calculated from Ka1:

[H⁺]²/([H₃PO₄] - [H⁺]) = Ka1

[H⁺]² = 0.00725*0.025 - 0.00725*[H⁺]

solving the equation, we obtain [H⁺] = 0.0103 M, [H₃PO₄] = 0.025-0.0103=0.015 M

The concentration of PO₄^3- is obtained from the product of acidity constants:

Ka1*Ka2*Ka3 = [H⁺]³*[PO₄^3-]/[H₃PO₄]

0.010³*[PO₄^3-]/0.015=1.82*10^-22

[PO₄^3-] = 2.73*10^-18 M