What are the mole fraction and the partial pressure of each gas?

Mole fraction vs. mole for gases?

• Kinda conceptual, not computational question. A previous question i got right was asking what the mole fraction of Hydrogen was in the container given the partial pressure of the gas at 5 atm. Was able to solve for that one pretty quickly. The next questione asked what the mass of CO2 would be given the partial pressure at STP was 30 torr. I tried solving this by figuring out the mole fraction of CO2 and multiplying by its molecular mass to solve for the Mass. I was dead wrong - the question doesn't have an answer due to incomplete data (missing volume). My question - I thought volume wouldn't be necessary given how the first question worked out. Why can't i find the mole fraction in this second problem and sub it in for the number of moles in PV=nRT.

• Answer:

  In the CO2 question, the mole FRACTION is not the same thing as the total number of moles. If you knew the actual number of moles of CO2, then you could get the mass of CO2 by multiplying the moles times the (mass per mole = 44). So you may have been right about the mole FRACTION of CO2, but that wouldn't help you with the total mass unless you knew something else - it wouldn't HAVE to be the total volume, it could be the mass of the other gases, or something else, but somehow you need to know if you're dealing with a pocket-sized flask or a blimp-sized tank.