Kc is equal to Kp when the number of moles of gas (n) doesn't change. If there is a liquid in a reaction, you use the vapor press for that entry in the Kp equation. Kp uses the same equation as Kc, but with pressure instead of concentration Solids and liquid are not included in the Kc and Kp equations because they have an activity equal to 1. Kc and Kp are the same thing as the k (rate constant) that you learned about before. Kc has units of molarity and Kp has units of pressure