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Air is made up of only 2 gases oxygen and nitrogen

  1. Oct 23, 2009 #1
    if air is made up of only 2gases oxygen and nitrogen, and the molecular mass of air is 29g/mol, find the % of each component in the air,

    what i thought i should do is make 2 equations,

    Mw(O)=16
    Mw(N)=14

    X*16 + Y*14 = 29 (in 1mol of air)
    X + Y = 1

    but the solution to this is not possible, i get one as a negative pct

    the correct answer is meant to be
    72.5%
    27.5%
     
  2. jcsd
  3. Oct 23, 2009 #2
    Re: chemistry

    These are atomic weights, not molecular weights.
    Remember oxygen and nitrogen are both diatomic gases.
     
  4. Oct 23, 2009 #3
    Re: chemistry

    thanks, so my equations would go

    X*2*16 + Y*2*14 = 29 (in 1mol of air)
    X + Y = 1


    then i get 75%, 25%
    is this correct? the "correct" answer in my book is 72.5% 27.5%
     
  5. Oct 23, 2009 #4
    Re: chemistry

    Is that 72.5% by weight, or by number of molecules?
     
  6. Oct 23, 2009 #5
    Re: chemistry

    it actually doesnt say, never thought of that,

    is what i got by mol;ecules? how would i make the conversioin
     
  7. Oct 23, 2009 #6
    Re: chemistry

    got it,,

    0.75*28=21g
    0.25*32=8g

    8/29=27.6%
    21/29=72.4%

    close enough
     
  8. Oct 23, 2009 #7

    Borek

    User Avatar

    Staff: Mentor

    Re: chemistry

    25% and 75% is a correct answer using 28, 32 and 29 as molar masses and assuming final result must be just molar fraction.

    27.6% and 72.4% is a correct answer in terms of mass percentage.


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