I was asked to give the equilibrium expression for:

$$\ce{CO2(aq) + H2O(l) <=> H2CO3(aq)}$$

As water is a liquid and a solvent here, it shouldn't be included in the expression, hence I wrote:

$$K_c = \frac{[\ce{H2CO3}]}{[\ce{CO2}]}$$

However, the answer of the question included water in the denominator.

What is the correct answer in this situation?


In the case of heterogenous equilibria when molar concentration of pure solid or liquid involved, water in this case is constant or is independent on the amount present its concentration is omitted as per the general norm. The equilibrium constant is just modified to

$$K_c' = K_c[\ce{H2O}]$$

So the book and you are referring to different constants.


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