Calculating equilibrium constant rules

[kasse]

Homework Statement

Find the eq. constant for the reaction

CH₃COOH (aq) + NO₂^- (aq) = CH₃COO^- (aq) + HNO₂ (aq)

The Attempt at a Solution

I find K_a for the reaction CH₃COOH = CH₃COO^- + H⁺ and K_b for the reaction NO₂^- + H⁺ = HNO₂

But what are the rules then? I try multiplying the constants, but this does not give the correct answer.

 

[eli64]

Homework Statement

Find the eq. constant for the reaction

CH₃COOH (aq) + NO₂^- (aq) = CH₃COO^- (aq) + HNO₂ (aq)

write the K expression for this equilibrium

then compare it to the Ka expressions for both CH3CO2H AND HNO2

 

[kasse]

I managed to find the correct solution by multiplying K_a1 and the 1/K_a2. Is 1/K_a2 the same av K_b?

 

[Borek]

I managed to find the correct solution by multiplying K_a1 and the 1/K_a2. Is 1/K_a2 the same av K_b?

No, they are related by Ka*Kb=Kw. Beware of how indices change - see very bottom of this page for details:

http://www.chembuddy.com/?left=pH-calculation&right=polyprotic-dissociation-constants

 

[eli64]

I managed to find the correct solution by multiplying K_a1 and the 1/K_a2. Is 1/K_a2 the same av K_b?

Did you just happen to work out the numbers and find the right answer or did you see that rearranging the Ka expressions for the acids gave Ka(CH3CO2)/Ka(HNO2) which was equal to the K expression for the original equilibrium?