PLEASE HELP?!? CHEM ? - The volume of one bike tire is 2.45L. Determine the mass savings...?

Question

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The volume of one bike tire is 2.45L. Determine the mass savings if a racer filled both of his tires with helium rather than air. Assume the temperature is 30 degrees C and the racer pumps his tires up to 125 psi (14.6956 psi=1atm). You can also assume that air is 80% nitrogen and 20% oxygen.

Answer 1

Molar mass of He: 4.003 g/mol
Molar mass of N2: 28.01 g/mol
Molar mass of O2: 32.00 g/mol
So the effective "molar mass" of air: 0.8*28.01 + 0.2*32.00 = 28.81 g/mol
With ideal gas formula, n = PV/RT = (125/14.7)*2.45/(0.08206*303) = 0.838 (mole).
Hence the mass saving is: 2*(28.81-4.003)*0.838 = 41.6 (g).