The reaction $\mathrm{A}(\mathrm{g}) \mathrm{f} \quad \mathrm{B}(\mathrm{g})+\mathrm{C}(\mathrm{g})$ was initiated with the amount ' a ' of $\mathrm{A}(\mathrm{g})$. At equilibrium it is found that the amount of $\mathrm{A}(\mathrm{g})$ remaining is ( $\mathrm{a}-x$ ) at a total pressure of p . The equilibrium constant Kp of the reaction can be calculated from the expression :