When the no. of moles of reactant gases (a + b) = the no. of moles of product gases (c + d).
Explanation:
The relation between Kp and Kc is:
Kp = Kc(RT)∧Δn.
So, Kp = Kc, when Δn = zero.
Δn is the difference between the no. of gas molecules in the products side and that in the reactants side.
So, Δn = zero, when the no. of molecules of gases in the products side = the no. of molecules of gases in the reactants side which means (a + b) = (c + d).