Question

Carbonic acid, H2CO3, has pka1 =6.40 and pka2 =10.20. Which species are present at the aqueous solution at the following pH's listed below?
a) 5.1
b) 6.4
c) 8.5
d) 13.5

Answer 1

Acid dissociation constant is the equilibrium constant for the dissociation reaction of acid. It gives the strength of the acid.

concentration of water can be ignored.

a.) pH = 5.1

The medium is acidic, the proton concentration is more so it will dissociate only once. The concentration of H₂CO₃ will be more than    (bicrbonate).

b.) pH = 6.4 = pKa1

So the concentration of   

Bythis equation for pH= pKa the concentration of HA and A^- should be equal.

c.) The medium is basic. So proton concentration is less. Thus second dissociation also occurs. There will be both present where bicarbonate will be more.

d.) The medium is strongly basic. Its above the pKa2 value. Thus the species completely dissociate into .