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Homework Help: Converting J/kg to kJ/mol

  1. Aug 18, 2011 #1
    Let's say I have a value of:

    553 x 10^5 j/kg

    How would I convert that to kJ/mol?
     
  2. jcsd
  3. Aug 18, 2011 #2

    rl.bhat

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    mol = mass/molecular weight. = kg/M
    kJ = 10^3 J
     
  4. Aug 18, 2011 #3
    so that means if I had 55300000 j/kg, that would be 55300 kJ/kg.

    How would I make it into kJ/mol....

    (sorry, i'm not very good at converting...)
     
  5. Aug 18, 2011 #4
    from J to kJ all you need to do is of course to devide in 1000 (=k).

    It's simply to look at it like this:

    10^3j = kj => j = 10^-3kj
    so 553 * 10^5 j = 553 * 10^5 * 10^-3 kj = 553 * 10 ^2 kj
    (I substituted j for kj using the upper formula).

    About kg=>mol, well, for that you'll have to tell us what's the material involved.
    Imagine there are plastic and metal balls. A plastic ball weighs 1 kg and a metal ball 2kg.
    It'd be like asking you "how many balls are in a sack weighing 10kg?". If you knew it's plastic balls, you'd say 10. If it were metal balls, you'd say 5.

    so 1kg can be 1000 mol and it can be 1 mol => we need more information.
     
  6. Aug 18, 2011 #5
    Well, this is actually a fuel, Kerosene, with a molecular weight of 120...
     
  7. Aug 18, 2011 #6
    Then use the relation given by rl.bhat and derive the connection between kg and mol. When you say 120, make sure if it's 120 g/mol or 120 kg/mol, and convert fittingly.
     
  8. Aug 18, 2011 #7
    So I have 55300 kJ/kg

    Assuming that my Molecular weight is 120g/mol.....

    Yeah, I have no idea. It seems like there's a missing variable to me :S
     
  9. Aug 18, 2011 #8
    How come?

    rl.baht wrote: mol = mass / molecular weight.

    we want to know the connection between kg and mol. So assume you have 1kg material.
    Step 1: convert your molecular weight from 120g/mol => X kg/mol (you do it :-)
    Step 2: use rl.baht connection written above, instead of mass you have "kg", and instead of molecular weight you have what you've calculated on Step 1.
    Step 3: from the formula of Step 2 isolate kg. You'll get something like kg = Y mol.
    Step 4: substitute kg for mol in the expression you've got.

    Give it a try, it's important and basic. If you won't make it I'll show you the way.
     
  10. Aug 18, 2011 #9
    Step One: 120g/mol => 0.12kg/mol
    Step Two: mol = mass/molecular weight
    = kg/0.12 = mol
    kg = 0.12mol

    So 55300kJ/0.12mol
    = 460833 kJ/mol

    Wow, that's a big number, did I do it right?
     
  11. Aug 18, 2011 #10
    hehe, the distance between the earth and the sun in centimeters is also a big number and it's still true :-)
    About the numbers - I really don't know about the 120 g/kg, you should make sure of that, but it seems you did it perfectly, great!
     
  12. Aug 18, 2011 #11
    Haha, thanks! :)
     
  13. Aug 19, 2011 #12

    rl.bhat

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    1 kg of kerosene produces 55300 kJ of energy.
    How much energy is produced by 0.120 kg? That is required energy per mol.
     
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