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Question

Two weak bases AOH and BOH having equal concentrations of 0.5 M are present in a solution. Calculate the ratio, [A+][B+] at equilibrium.
Dissociation constants for AOH and BOH are 1.1×108 and 1.7×109 respectively.(Report the answer upto second decimal)

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Solution

For AOH :

AOH(aq)OH(aq)+ A+(aq)

at t=0 C1 0 0

at t=teq C1C1α1 (C1α1+C2α2) C1α1

For BOH :

BOH(aq)OH(aq)+ B+(aq)

at t=0 C2 0 0

at t=teq C2C2α2 (C1α1+C2α2) C2α2

Dissociation constant for AOH Kb1 :
Kb1=(C1α1+C2α2)(C1α1)C1(1α1)
Dissociation constant for BOH Kb2 :
Kb2=(C1α1+C2α2)(C2α2)C2(1α2)
Since α1 and α2 are very small in comparison to unity for weak bases.So 1α11 and 1α21.
Kb1=(C1α1+C2α2)(C1α1)C1(1α1)

Kb1C1=(C1α1+C2α2)(C1α1)
Similarly,
Kb2C2=(C1α1+C2α2)(C2α2)
Now,
Kb1C1Kb2C2=(C1α1+C2α2)(C1α1)(C1α1+C2α2)(C2α2)Kb1C1Kb2C2=(C1α1)(C2α2)=[A+][B+][A+][B+]=Kb1C1Kb2C2=1.1×108×0.51.7×109×0.5[A+][B+]=6.47

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