You have a 1 L container of a gas O 2 at 20°C and 1 atm. Subtracting the weight of the container, what is the mass of the gas?
PV = nRT and solve for n = number of mols.
Then n = grams/molar mass. You know n and molar mass; solve for grams.
Then n = grams/molar mass. You know n and molar mass; solve for grams.