5.00 liter steel cylinder is filled with water vapor at 100 °C and 650 mm Hg.

Determine the mass of water that condenses to a liquid when the closed
system is cooled to 20 °C. The vapor pressure of water at 20 °C is 18 mm
Hg.
6.

You know how much water vapor is in the steel cylinder initially (I worked that for you last night). It is

n = PV/RT and I think the answer is about 0.14 moles or about 2.5 g H2O vapor.

So if you cool the cylinder to 20 C, the only gas (water vapor) will be that due to the vapor pressure of the H2O (18mm); therefore, what will happen if you use 18 mm as P and 293 as T (in kelvin) and solve for n. Then grams water vapor = n x 18 and subtract from the original to find how much condensed.