Question

A gaseous mixture of O2 and N2 contains 31.8 % nitrogen by mass. What is the partial pressure of oxygen in the mixture if the total pressure is 525 mmHg ?

Answer 1

mole fraction O2 = moles O2 / total moles in the system

Then partial pressure O2 = mole fraction O2 x total pressure


let us fix 100 g of sample
mass N2 in 100 g = 31.8 % x 100 g = 36.8 g
moles N2 = mass / molar mass = 31.8 g / 28.02 g/mol = 1.1349 mol

% mass O2 = 100 % - 31.8 % = 68.2 %
mass O2 in 100 g = 68.2 g
moles O2 = 68.2 g / 32.00 g/mol = 2.13125 mol


total moles gas = 1.1349 mol + 2.13125 mol = 3.26615 mol

mole fraction O2 = 2.13125 mol / 3.26615 mol
= 0.6006

partial pressure O2 = 0.65252 x 525 mmHg
= 342.576mmHg
= 342mmHg ( 3 sig figs)