Calculate the mol fraction

Question

A 2.50L container at 17.0 °C contains a mixture of three gases, N₂, He, and CO₂, at partial pressure of 0.32 atm, 0.25 atm and 0.4 atm respectively. 

 

a) calculate the total pressure of the gas mixture.

 

What I've done for this is added up all the gas pressures to total 0.97 atm.

 

b) Calculate the mol fraction of CO₂

This is where I need help, do I use PV=nRT to find the moles of CO₂ ? and if so, what would be the next step?

Answer 1

Haley,

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,

X_N2 = 0.32 atm/0.97atm = 0.33

X_He = 0.25 atm/0.97 atm = 0.26

X_CO2 = 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.