A 1.87-L flask is filled with propane gas (C3H8), at 1.00 atm and -22.1°C. What is the mass of the propane in the flask?

Use PV = nRT

P = 1 atm
V = 1.87 L
n = mols. Solve for this
R = 0.08206 L*atm/mol*K
T = you want Kelvin. That is K = 273.2 + (-22.1) = ?
That gives n = mols. Then
mol = grams/molar mass. You have mols and molar mass, solve for grams.
Post your work if you get stuck.