Question

Calculate pka1 , pka2, pka3 for Arsenic acid (H3AsO4) using Paulings rules. Are numbers close to experimental values? Explain.

Answer 1

Pauling's first rule for predicting the pKa of a mononuclear oxoacid is pKa = 8 - 5p (where p represents the number of oxo
groups attached to the central element).
Since p = 1, the predicted value of pKa(l) for H3AsO4 is 8 - 5 x 1 = 3.
Pauling's second rule for predicting the pKa of a mononuclear oxoacid is that successive pKa
values for polyprotic acids increase by five units for each successive proton transfer. Since
pKa1 for H3AsO4 was predicted to be 3

pKa2 = 3+1*5 = 8

pKa3 = 3+2*5 = 13

The original values are :

pKa1 = 2.22, pKa2 = 6.98, and pKa3 = 11.53.

So there pKa values are differed nearly by 1.5 units...may be approximately correct