Question

Helium-oxygen mixtures are used by divers to avoid the bends and are used in medicine to treat some respiratory ailments. What percent (by moles) of He is present in a helium-oxygen mixture having a density of 0.498 g/L at 25 ∘C and 721 mmHg?

Answer 1

H2-O2

% of He in mix if:

D = 0.498 g/L

at

T = 25°C = 298 K, P = 721 mm Hg = 721/760 = 0.9486 atm

so

PV = nRT

n = PV/(RT)

assume V = 1 L

n = (0.9486)(1)/(0.082*298) = 0.038819

mass = 0.498 g

so

MW = mass/mol = 0.498/0.038819 = 12.828 g/mol

so

MW avg = x-He* MW He +  x-O2* MW O2

substitute data

12.828 = x * (4) + (1-x) * 32

solve for x

12.828 = 4x + 32 - 32x

32x-4x = 32 - 12.828   

x = (32 - 12.828 ) / (32-4)

x = 0.68471of He

so

% --> 0.68471 * 100% = 68.47%