Define molar volume for a gas at STP. Define molar volume for a gas at STP. The molar volume of an ideal gas is the volume occupied by one mole of gas at T = 0 ∘C (298 K) and P = 1.00 atm. The molar volume of an ideal gas is the volume occupied by approximately one mole of gas at T = ∘C (273.15 K) and P = 1.00 atm approximately. The molar volume of an ideal gas is the volume occupied by exactly one mole of gas at T = 0 ∘C (273.15 K) and P = 1.00 atm exactly. The molar volume of an ideal gas is the volume occupied by exactly one mole of gas at T = 25 ∘C (298.15 K) and P = 1.00 atm exactly.