Consider a binary vapor mixture of two non-polar compounds at sub-critical pressures. What will be the most reasonable result of modeling the system pressure from an equation of state given a particular temperature and molar volume?

Question

A. The predicted pressure from the VDW equation of state will be smaller than the ideal gas pressure.
B. The predicted pressure from the VDW equation of state will be larger than the ideal gas pressure.
C. You will obtain three pressure values from the cubic equation of state at a given temperature and molar volume. Thus, you choose the largest value since you are interested in the vapor phase.
D. You will obtain three pressure values from the cubic equation of state at a given temperature and molar volume. Thus, you choose the smallest value since you are interested in the vapor phase.
E. None of the above are true.

Answer 1

A. Correct. At sub-critical states, the impact of the attractive term in the equation of state has a larger impact than the repulsive term. Thus, the pressure will be less than what is predicted by the ideal gas law.
B. Incorrect. Review the terms in the VDW equation of state to help make your decision.
C. Incorrect. At a given molar volume and temperature, there will be only one pressure value. It is the molar volume that has three solutions for a given T and P.
D. Incorrect. At a given molar volume and temperature, there will be only one pressure value. It is the molar volume that has three solutions for a given T and P.
E. Incorrect. One statement is correct.