Consider the equations given below
`"Ca CO"_(3(s))hArrCaC_((s))+CO_(2(g))`
`CO_(2(g))+H_(2)O_((e))hArrH_((aq))^(+)+HCO_(3(aq))^(-)`
Write the equilibrium constant for these equations and give reason for the exception of concentration of specific compounds.

Explanation

A pure solid always has the same concentration at a given temperature, as it does not expand to fill its container. (i.e.) it has same number of `"moles L"^(-1)` of its volume. Therefore, the concentration of a pure solid is constant. The above expression can be modfied as follows.
`K_(c)=[CO_(2)(g)]` or `K_(P)=P_(CO_(2))`
The equilibrium constant for the above reaction depends only the concetration of carbon dioxide and not the calcium carbonate or calcium oxide. Similarly, the active mass (concentration) of the pure liquid does not change at a given temperature. Consequently, the concentration terms of pure liquids can also be excluded from the expression of the equilibrium constant.
For example,
`CO_(2)(g)+H_(2)O(l)hArrH+(aq)+HCO_(3)^(-)(aq)`
Since, `H_(2)O(l)` is a pure liquid the `K_(c)` can be expressed as
`K_(c)=([H^(+)(aq)][HCO_(3)^(-)(aq)])/([CO_(2)^(-)(g)])`