The partial pressure of a gas is the pressure that is imposed upon the wall of the vessel by the gas, if the gas was present by itself and  allowed to occupy the same volume as that of the mixture. It is expressed as the product of mole fraction of the gas and total pressure of the mixture.

Gas-Phase Equilibrium Constant:

An alternative equilibrium constant is available in reversible reaction systems that have at minimum one gaseous component. It is identified as Kp, with the “p” subscript indicating that it is computed from partial pressure values. In particular it is calculated using an array that contains non-zero, constant pressures for each gaseous species in equilibrium at a certain constant temperature. Like the more common Kc equilibrium constant, the magnitude of Kp is equal to a reaction quotient, that is calculated using a mathematical formula relating to the various partial pressure values.