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Confused about mole ratio being relative or absolute

  1. Mar 1, 2013 #1
    Let's start with a generalized example:

    3A + 1B -> 2C

    For the mole amount next to each molecule, am I always to consider these as relative to one another or absolute? Most of the videos I have seen are describing the above as "for every 3 moles of A and 1 mole of B you get 2 moles of C". I believed at some point that these numbers were absolute however, after reading up on equilibrium I discovered that I was incorrect.

    So for a reaction such as this:


    I have a ratio of 2:1:3. If I used up 0.8 moles of the nitrogen, I read that I would use the ratio to discover that I would produce 0.4 moles of nitrogen and 1.2 moles of hydrogen. Would anyone care to explain this to me?
  2. jcsd
  3. Mar 1, 2013 #2
    Correct. A molar ratio is just that, a ratio. It's a ratio in the same way that 1:2 is the same as 0.5:1 or 2:4. Just because the reaction says 2NH3 doesn't mean you have to use 2 moles.
  4. Mar 7, 2013 #3


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    Gold Member

    .8NH3 x 1N2/2NH3= .4N2
    .8NH3 x 3H2/2NH3= 1.2H2
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