chemistry question I dont get

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 0.518 g L−1 at 25 ∘C and 721 Torr?

Express your answer numerically as a percentage.

please help I do not get this show all the work please I'm confuse

Answer 1

assume 1 liter of the gas mixture.

0.518 g / L x 1 L = 0.518 g of gas

Finding moles of gas, using the ideal gas law we have...

PV = nRT

solve for n (moles of gas)

Let x = mols of one gas n - x = mols of other gas.

Convert moles to grams of each and set sum of grams equal to 0.518 g

Convert back to moles and take ratio of mols He to total moles to find the answer