Question

What is the mass of 5.00 L of propane vapor (C₃H₈) at STP?

Answer 1

PV = nRT
where P = pressure, V = volume, n = moles, R is the gas constant, and T is temperature

you are given STP, which implies
P = 1 atm
T = 273K

you are also given V = 5.00L from the problem

since we are using units of atm, moles, L, K then R = 0.082057

so you can plug everything in and solve for n. to go from moles to grams, simply multiply by the molar mass of propane.