J.R. S. answered 01/30/18
Ph.D. University Professor with 10+ years Tutoring Experience
So far, you're on the right track. You do add all the pressure to get the total pressures. Now, according to Dalton's Law of Partial Pressure, mole fraction of a gas (X) = partial pressure of X/total pressure. You could use PV = nRT and calculate moles of each gas, and then find mole fraction of each, but it's much easier/faster to use Dalton's Law. Thus,
XN2 = 0.32 atm/0.97atm = 0.33
XHe = 0.25 atm/0.97 atm = 0.26
XCO2 = 0.4 atm/0.97 atm = 0.41
You should go ahead and use PV = nRT and see if you don't get the same mole fractions.
Hope this helps.
Thank you for the explanation. Dalton's Law is definitely faster/easier. I did use the Idea Gas Equation to double check, and got the same answer. Thanks for the help!