Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Unit Conversions: kJ/mol to kJ/litre

  1. Aug 24, 2011 #1
    Let's say I have 4163 kJ/mol of energy coming from a substance with molar mass 86.14g/mol. How would I convert the 4163 kJ/mol of energy to kJ/litre?

    Thanks.
     
  2. jcsd
  3. Aug 24, 2011 #2

    gneill

    User Avatar

    Staff: Mentor

    You'll need to also know the density (g/L) of the substance.
     
  4. Aug 24, 2011 #3
    The density of this substance is 0.6548 g/mL.
     
  5. Aug 24, 2011 #4

    lightgrav

    User Avatar
    Homework Helper

    your answer wants kJ on top, so you start with 4163 kJ/mol .
    You multiply by a number that has moles on top , so they cancel.
    but now there's something else on the bottom,
    so you multiply by a number that has grams on top . then you're done.
     
  6. Aug 25, 2011 #5
    But I don't have a number with mols on top o_o
     
  7. Aug 25, 2011 #6

    Borek

    User Avatar

    Staff: Mentor

    Conversion factor

    [tex]\frac {86.14 g} {1 mol}[/tex]

    can be reversed to

    [tex]\frac {1 mol} {86.14 g}[/tex]

    The idea is that conversion factor equals 1, switching nominator and denominator doesn't change the value, although it moves units.
     
  8. Aug 25, 2011 #7
    That means, I can multiply 4163kj/mol by 1/86.14g, which then I get 4163kJ/86.14g.
    Then I multiply this number by the density. So 4163kJ/86.14g x 0.6578g/mL
    which equals to 31.64 kJ/mL

    Is that right?
     
  9. Aug 25, 2011 #8

    Borek

    User Avatar

    Staff: Mentor

    To be precise by 1 mol/86.14g, but I guess it is just a typo.

    OK so far, but you are not there yet - you need to convert it to kJ/L. But that's the simplest part.
     
  10. Aug 25, 2011 #9
    Don't I just multiply it by 1000? So I get 31645.4 kJ/L.

    But the thing is, this substance is Hexane, and according to Wikipedia, it has a heat of combustion of 29.3 kJ/L. The two numbers aren't even remotely close.
     
  11. Aug 25, 2011 #10

    Borek

    User Avatar

    Staff: Mentor

    Yes.

    Obviously what they mean is MJ/L, otherwise their numbers are not consistent. As much as I like wikipedia, it is hardly a definite source.
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook