Posted by Kathy on .
A mixture of He and CO2 in a flask at 45.0°C exerts a pressure of 955 torr. All of the CO2 is removed and is found to weigh 0.879 g. The pressure drops to 555 torr after the removal of CO2. What is the mass of He?

Chemistry 
DrBob222,
You don't have a volume listed but you can calculate it.
PCO2 = 955555 = ?? mm Hg
PHe = 555 mm Hg.
Use PCO2 and PV = nRT to solve for V. Then use PV = nRT and PHe to solve for nHe, then n He= grams/molar mass.