Question

pKalpKa2 2.16 7.21 pKa3 12.32 Phosphoric acid, H2PO4 (aq), is a triprotic acid, meaning that one molecule of the acid has three acidic protons. Estimate the pH and the concentrations of all species in a 0.400 M phosphoric acid solution. [H2PO4) = (H+) =( M M 7m [H, PO1 = [OH-] = M [HPO4) = MpH = [PO] = M

Answer 1

[H₃PO₄] = 0.351 M

[H₂PO₄^-] = 0.0493 M

[HPO₄^2-] = 6.17 x 10^-8M

[PO₄^3-] = 4.79 x 10^-13 M

[H⁺] = 0.0493 M

[OH^-] = 2.03 x 10^-13M

pH = 1.31

Explanation

pK_a1 = 2.16

K_a1 = 10^-pK_a1

K_a1 = 10^-2.16

K_a1 = 6.92 x 10^-3

initial concentration H₃PO₄ = 0.400 M

ICE table	H3PO4	$\rightleftharpoons$	H2PO4-	H+
Initial conc.	0.400 M		0	0
Change	-x		+x	+x
Equilibrium conc.	0.400 M - x		+x	+x

K_a1 = [H₂PO₄^-]_eq[H⁺]_eq / [H₃PO₄]_eq

6.92 x 10^-3 = [(x) * (x)] / (0.400 M - x)

Solving for x, x = 0.04926 M

[H⁺] = x = 0.04926 M

pH = -log[H⁺]

pH = -log(0.04926 M)

pH = 1.3075