Themes > Science > Chemistry > Inorganic Chemistry > More Information About Gas Laws > Gas Index > Mole Fraction

The mole fraction of a gas in a gas mixture is just the numebr of moles of that gas divided by the total number of moles of all the gases in the mixture
mole fraction = Xa = na/ntot

The partial pressure of a gas in a mixture is equal to the mole fraction of that gas multiplied by the total pressure of the gas.

Pa = XA*Ptot

Example:What is the partial pressure of 0.50 moles of nitrogen in a sample of gas that has a total pressure of 6.34 atm and 1.39 total moles of gas?

Solution: First, work out the mole fraction of nitrogen in the sample

XN2 = nN2/ntot
XN2 = 0.50/1.39
XN2 = 0.36
Now just multiply the mole fraction by the total pressure to get the partial pressure of nitrogen
PN2 = XN2*Ptot
PN2 = 0.36*6.34 atm = 2.3 atm


Information provided by: http://learn.chem.vt.edu