In: Chemistry
2. For the diprotic weak acid H2A, Ka1 = 2.7 × 10-6 and Ka2 = 8.8 × 10-9. What is the pH of a 0.0500 M solution of H2A? What are the equilibrium concentrations of H2A and A2– in this solution?
H2A ---------> HA- + H+
I 0.05 0 0
C -x +x +x
E 0.05-x +x +x
ka1 = [H+][HA-]/[H2A]
2.7*10-6 = x*x/0.05-x
2.7*10-6 *(0.05-x) = x2
x = 0.000366
[H+] = x = 0.000366M
[HA-] = x = 0.000366M
HA- -------> A2- + H+
Ka2 = [A2-][H+]/[HA-]
8.8*10-9 = [A2-]*0.000366/0.000366
[A2-] = 8.8*10-9 M
PH = -log[H+]
= -log0.000366 = 3.4365