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Homework Help: A simple unit conversion problem that I don't get.

  1. Apr 1, 2007 #1
    The question is a enthalopy problem where the enthalopy has to be caluculated per mole at the end. I understand how to calculate the enthalopy change, but I can't seem to calculate the number of moles in the question to be able to calculate the change in enthalapy per mole. I'm given 50.0 cm^3 of 1.00 mol dm^-3 of a substance. The answer is given so I know it's 0.05 moles but how was that calculated? And also why is it dm^-3 and not dm^3?

    I know that a cm is a hundreth of a meter and a dm is 10cm/meter or is it? I'm really confused about my units (especially the cubed parts) so if someone can clarify all this it would be greatly appreciated. :smile:
  2. jcsd
  3. Apr 1, 2007 #2


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    You are given a molar “density” (moles per unit volume) of your substance as well as the volume of the substance used in the reaction.

    Your molar density is given as,
    1.00 mol dm^-3

    Now you asked why it is dm^-3 and not dm^3….maybe it is because of this fact that you did not realize you were given a density.
    Remember the ‘rules of exponents’…a negative exponent in the numerator of a fraction can be re-written as a positive exponent in the denominator of a fraction. So if you were to move the dm term to the denominator of the fraction you would get,
    1.00 mole / dm^3
    And of course dm^3 is a unit of volume equal to 1000 cubic centimeters (10 cm * 10 cm * 10 cm = 1000 cm^3 = 1 dm^3).

    So now you know how many moles of your substance are in a given volume, and you also know the volume (50.0 cm^3), so you should be able to calculate the number of moles.
  4. Apr 1, 2007 #3
    Still a bit confused...

    You're right I forgot about the rule of exponents, but I'm still a bit confused about "And of course dm^3 is a unit of volume equal to 1000 cubic centimeters (10 cm * 10 cm * 10 cm = 1000 cm^3 = 1 dm^3)." :confused: can anyone simplify this concept any further?
    P.S. Is 1dm = 10cm ?
  5. Apr 2, 2007 #4


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    The prefix "deci-" (abbreviated "d" here) means one tenth. Deciliter is 100 mL and decimeter is 10 cm.
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