Question

Using Pauling’s rules, calculate the pKa values (all) of following acids: (PKa1,Pka2 or pka3 if acid is polyprotic)

H3PO4, H3PO3 and H3PO2

Answer 1

Pauling's rule predict the pKa of acids based on the number of oxo groups and hydroxyl group in the acid.

Rule 1

If an oxoacid of element E can be written as , its pKa is .

Rule 2

The successive pKa value of polyprotic acids that have more than one OH increases by 5 unit for each proton transfer.

Now, the structure of the given acids are

For

It can be written as . Hence, it is a polyprotic acid with 3 abstractable protons.

Hence, x = 1, y = 3.

Thus the pKa1 =

Applying rule 2,

Similarly

Now for

It can be written as . It has two abstractable protons.

Hence, x=1, y=2.

pKa1 =

Now for

It can be written as .It is not a polyprotic acid. Hence, it only has one pKa value.

x=1,y=1.