Question

Find the mole fractions and partial pressures in a 12.5 L tank with 24.2 g He and 4.32 g O2 at 298 K

Answer 1

you are given the masses of each gas, so start by working out moles of each gas

moles = mass / molar mass
moles O2 = 4.32 g / 32.00 g/mol = 0.135 mol
moles He = 24.2 g / 4.00 g/mol = 6.05 moles

mole fraction O2 = moles O2 / total moles in system
= 0.135 mol / 6.185 mol = 0.0218

Then do the same for He

You can ten use the total moles to calculate the total pressure in the system using
PV = nRT
P = Pressure = ? atm
V = volume = 12.5 L
n = total moles = 6.185 mol
T = temp in Kelvin = 298 K
R = gas constant = 0.082057 L atm mol^-1 K^-1

P (tot) = nRT / V
= 6.185 mol x 0.082057 x 298 K / 12.5 L
= 12.1 atm