A volume of 18.0 L contains a mixture of 0.250 mole N2 , 0.250 mole O2 , and an unknown quantity of He. The temperature of the mixture is 0 ∘C , and the total pressure is 1.00 atm . How many grams of helium are present in the gas mixture?

Answer 1

Use PV = nRT and solve for n = total mols.
nToptal = nN2 + nO2 + nHe. nHe is the only unknown, solve for that.
Then nHe = grams/molar mass
You know nHe and molar mass, solve for grams.
BTW, help is not the subject. Usually I pass these up because they don't say chemistry.