The partial pressure of a gas is the pressure that is due to that particular gas only. Recall Avogadro's hypothesis that equal volumes of gases contain the same number of moles. The mole fraction of the gas will be 
the partial pressure = mole fraction × the total pressure.The equilibrium law in terms of Kp for the reactionaA + bB ⇌ cC + dD
the partial pressure = mole fraction × the total pressure.The equilibrium law in terms of Kp for the reactionaA + bB ⇌ cC + dD