A mixture comprised of 30-mol-% methane, 10-mol-% ethane, 30-mol-% propane, and 30-mol-% n-butane is brought to a

Question:

A mixture comprised of 30-mol-% methane, 10-mol-% ethane, 30-mol-% propane, and 30-mol-% n-butane is brought to a condition of - 15°C at pressure P, where it exists as a vapor/liquid mixture in equilibrium. If the mole fraction of the methane in the vapor phase is 0.80, what is pressure P fin bar)?