# A simple unit conversion problem that I don't get.

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. Related Biology and Chemistry Homework Help News on Phys.org
mrjeffy321
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.

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)." can anyone simplify this concept any further?
P.S. Is 1dm = 10cm ?

chemisttree