I have an exam in a day and I'm confused on how to solve this question. Do I use PV=nRT? Do I find out the molecular formula of the gas from the percentage of nitrogen and oxygen?

A sample of a mixture of gases contains 80.0% nitrogen gas and 20.0% oxygen gas by volume. Calculate the mass of 1.00 L of this mixture at STP.

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asked by Jenny
  1. I think the easiest (but the long) way is to say volume N2 = 0.8 x 1 L = 0.8 L
    volume O2 = 0.2 x 1L = 0.2 L.
    Then use PV = nRT and solve for n = mols N2 and mols O2.
    Then mols N2 = grams/molar mass
    and mols O2 = grams/molar mass. You know molar mass and mols, solve for grams each.

    A second way is to find the molar mass of the gas that is 80% N2 and 20% O2.
    That will be (0.8*28)+(0.2*32)=? g/mol and since 1 mol occupies 22.4L, then 1 L will be ?/22.4 = x?
    I did both and obtained the same answer for both.
    and ?/22.4

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