[alex]

My chemistry teacher has told us that for the reversible reaction: aA +bB <---------> cC + dD the equilibrium constant for the reaction can be calculated using the following equation:
 K_c = [C]^c[D]^d / [A]^ab
He then said that 'the proof is faulty but the answer is correct.' Could someone explain what is faulty about the proof.

Thanks

[Borek]

What proof of what?

[alex]

I got the impression that he was saying that the equilibrium constant for the reaction, that is calculated using the equation, is the correct value however there is actually an error in the calculations which are done to arrive at the answer.

[Borek]

Perhaps he was referring to the kinetic derivation that is faulty, but that's not the only way of arriving to the final expression.

However, the equation - as given - IS faulty by itself, as it contains concentrations. It should contain activities. For very diluted solutions it doesn't matter (both are identical), but for 0.01M solution of salts (ie solutions containing ions) the difference starts to play important role, for 0.1M solutions it can't be neglected and for 1M solution results can be completely off.

http://www.chembuddy.com/?left=pH-calculation&right=ionic-strength-activity-coefficients

[alex]

the equation - as given - IS faulty by itself, as it contains concentrations. It should contain activities.

I think this is what he was referring to. Thanks for your help.