A light hydrocarbon mixture containing 5 mol-% propane. 85 mol-% n-butane, and 10 mol-% n-pentane is stored

Question:

A light hydrocarbon mixture containing 5 mol-% propane. 85 mol-% n-butane, and 10 mol-% n-pentane is stored as a liquid under its vapor pressure in a refrigerated tank. Available refrigeration keeps the contents of the tank at 40(°F) Although the tank contains ample vapor space, it is essentially full of liquid on a molar basis. What is the storage pressure?