Question

A 2.30 L sample of O2(g) was collected over a liquid at a total pressure of 785 torr and 25°C. When the O2(g) was dried (vapor of that liquid removed), the gas had a volume of 1.93 L at 25°C and 785 torr. Calculate the vapor pressure of this liquid at 25°C.

Answer 1

Use PV = nRT
Solve for n=PV/RT for the first set of conditions which will give you total n (moles vapor + moles O2).

Then calculate n = PV/RT for the second set of conditions which gives just moles O2.
Subtract to find moles liquid vapor, then use PV = nRT and solve for P of the liquid (vapor of the liquid).