You have two flasks of equal volume. Flask A contains H2 at 0°C and 1 atm pressure. Flask B contains CO2 gas at 0°C and 2 atm pressure. Compare these two gases with respect to each of the following

The long way, which may be the best for beginners, is to use PV = nRT. Substitute and solve for n = number of mols; then mols = grams/molar mass. You know mols and molar mass, solve for grams and you have your answer.