Carbonic acid H2CO3 is a diprotic acid with Ka1 42 x 107 an

Carbonic acid, H2CO3, is a diprotic acid with Ka1 = 4.2 x 10-7 and Ka2 = 4.7 x 10-11. What is the equilibrium constant for the reaction:
CO32-(aq) + H2O (aq) HCO3- (aq)+ OH-(aq)

2.4 x 106

2.4 x 10-8

2.0 x 10-17

2.1 x 1010

2.1 x 10-4

Solution

the given reaction is

C032- + H2O <---> HCO3- + OH-

this reaction represents the Kb of C032-

now

the first Ka1 value represents dissociation of H2C03

the second one , Ka2, value represent dissociation of HCO3-

the conjugate base of HCO3- is CO32-

now

we know that

Ka x Kb = 10-14

4.7 x 10-11 x Kb = 10-14

Kb = 2.127 x 10-4

so

the answer is 2.1 x 10-4