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Homework Help: # of atoms when combining elements

  1. Nov 4, 2012 #1
    1. The problem statement, all variables and given/known data

    One liter of nitrogen combines with 3 L of hydrogen
    to form 2 L of ammonia. If the molecules of nitrogen
    and hydrogen have two atoms each, how many atoms of
    hydrogen and nitrogen are in one molecule of ammonia?

    2. Relevant equations

    Chemical equations

    3. The attempt at a solution

    I didn't know where to start so I looked at the solution key and got this:

    The number of liters tells us how many molecules of the original combine to form this number of molecules of the product. Using these numbers as multipliers in a chemical equation gives us

    1N2 + 3H2 → 1Nx Hy

    Making sure that we have the same number of atoms of each kind on each side, we get x = 1 and y = 3.

    Can somebody explain to me what the hell just happened?
  2. jcsd
  3. Nov 4, 2012 #2
    Try asking in the "Other Science" homework forum. While most of us here are capable of helping, this is a chemistry, not physics question.

    As for what happened:
    What is the relation between number of molecules and volume, given constant pressure and temperature?
  4. Nov 4, 2012 #3


    User Avatar

    Staff: Mentor

    You have two liters of ammonia, not one. So something is wrong with the right-hand side of the reaction as you've written it. Fix that and it will make sense.
  5. Nov 4, 2012 #4
    Sorry, this question came from my physics HW so I posted it here.
    Maybe a moderator can move it.

    Even when I change it to 2L it makes no sense to me. I haven't taken chemistry since high school.
  6. Nov 4, 2012 #5
    There are the same number of nitrogen molecules in 1L of nitrogen as there are hydrogen molecules in 1L of hydrogen and ammonia molecules in 1L of ammonia.
    A nitrogen molecule is composed of two nitrogen atoms and a hydrogen molecule is composed of two hydrogen atoms. An ammonia molecule is composed of (for you) an unknown number of nitrogen and hydrogen atoms. That's where x and y came from.
    You know that 1 part nitrogen molecule and 3 parts hydrogen molecule becomes 2 parts ammonia molecule. So [itex]1\times 2[/itex] parts nitrogen atoms becomes [itex]2\times x[/itex] nitrogen atoms and [itex]3\times 2[/itex] parts hydrogen atoms becomes [itex]2\times y[/itex] hydrogen atoms.
  7. Nov 4, 2012 #6
    Aaaah I see! Much appreciated Sir!
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