With this cubic virial equation, the coefficients B and C can be solved in closed form. Imposing the critical conditions: d P d v = 0 and d 2 P d v 2 = 0 the cubic virial equation can be solved to yield: B = − − v c , C = v c 2 3 , and Z c = P c v c R T c = 1 3 . Z c is therefore 0.333, compared to 0.375 from the Van der Waals equation.