A. can be found easily from the periodic table
B. is the mass of a mole of the gas
Step-by-step explanation:
Atomic masses of all elements are mentioned in the periodic table. From that we can find out the molar mass of a gas.
Molar mass (M) is equal to the mass of one mole of a particular element or compound; as such, molar masses are expressed in units of grams per mole (g mol–1) and are often referred to as molecular weights.