In: Chemistry
For the diprotic weak acid H2A, Ka1 = 3.1 × 10-6 and Ka2 = 6.6 × 10-9. What is the pH of a 0.0700 M solution of H2A? What are the equilibrium concentrations of H2A and A2– in this solution?
pH=
[H2A]=
[A^2-]=
H2A --------------> H^+ (aq) + HA^-
I 0.07M 0 0
C -x +x +x
E 0.07-x +x +x
Ka1 = [H^+][A^-]/[HA]
3.1*10^-6 = x*x/0.07-x
3.1*10^-6*(0.07-x) = x^2
x = 0.000464
[H^+] = 0.000464M
PH = -log[H+]
= -log0.000464
= 3.3334
[H2A] = 0.07-x =0.07-0.000464 = 0.0695M
[HA^-] = x = 0.000464M
HA^- ----------> A^2- + H^+
Ka2 = [A^2-][H^+]/[HA^-]
6.6*10^-9 = [A^2-]*0.000464/0.000464
[A^2-] = 6.6*10^-9M