The data that was collected allowed for the ideal gas formula to be used. The pressure was able to be determined from by subtracting the water vapor pressure from the total atmospheric pressure. The volume of the Butane gas used was able to be determined by subtracting the final volume of the jar from the initial volume of the jar. Use this formula in the homework problem to find the molar mass of the gas to see if it matches the molar mass of CO 2. Finding the Density of a Gas at Temperatures Different than O°C. When you calculate the molar mass of a gas using the Periodic Table, this gives you the weight of one mole of that gas, which would have a volume of 22.4 liters. The Combined Gas Law Formula is the relationship of changing pressure, temperature, and volume of an ideal gas. The same amount of the same gas is given at two different sets of conditions. Let's call the first set of measurements, 'condition #1,' and the second set of measurements, 'condition #2.' temperature and pressure remain constant. Thus when the container is weighed again there is air missing that needs to be counted in the final mass when the mass of the condensed liquid is determine by difference. This results in an apparent mass of liquid that is too small, which in turn makes the formula weight too small.