Question

Calculate the pH of a polyprotic acid give and sketch the titration curve for the following reaction: A 10.0-mL aliquot of 0.100M Na3AsO4 titrated with 0.100M HCl. pK1 = 2.25, pK2 = 6.77, pK3 = 11.60.

Answer 1

2)

upon adding the first 10 ml of HCl to the Na3ASO4, you are at the first equiv point having prorduced the acid HASO4)-2 whose pKa is 11.60

upon adding the second 10 ml of HCl to the Na3ASO4, you are at the second equiv point having prorduced the acid H2ASO4)-1 whose pKa is 6.77


upon adding the third 10 ml of HCl to the Na3ASO4, you are at the third equiv point having prorduced the acid H3ASO4)-1 whose pKa is 2.25

at this point you have 0.100 Molar Na3AsO4 converted to H3ASO4 but diluted:
0.100 Molar @ 10 ml / 40 ml total = 0.025 Molar H3AsO4

and this acid dissociates:
H3AsO4 --> H+ & AsO4)-2
0.025 -x --> x & x

Ka1 = [H+] [H2AsO4)-2] / [H3AsO4]

& for a pKa = 2.25, the Ka = 5.62e-3

5.62e-3 = [x] [x] / [0.025-x]

X2 = ( 5.62e-3) [0.025-x]

X2 = 1.4e-4 - 5.62e-3x

X2 - 1.4e-4 + 5.62e-3x = 0

x = [H+] = 0.0093 molar

pH = 3.85