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Old Sep 28th 2017, 11:58 PM   #1
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mole calculation.

This question is problem 1.3.1 from Herbert Callen's textbook in Thermodynamics, second edition.

> One tenth of a kilogram of NaCl and 0.15 kg of sugar ($C_{12} H_{22}O_{11}$ are dissolved in 0.5 Kg of pure water. The volume of the resultant thermodynamic system is $0.55\times 10^{-3}m^3$. What are the mole numbers of the three components of the system? What are the mole fractions? What is the molar volume of the system?

In the answers I have the following: Mole fraction of NaCl = 0.057, molar volume =$18\times 10^{-6} m^3/mole$.

How did they arrive at these answers? did they multiplied the ratios with Avogadro's number?
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Old Sep 29th 2017, 05:05 AM   #2
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Perhaps more of a Chemistry problem than Physics, but...

The key to this is converting the masses of each compound into an estimate of the number of atoms of each compound.
Any good depiction of the periodic table will include the atomic weights for each element.
These are the number of grams of each element that contain (approximately) 1 mole of atoms.
(A mole is just another name for Avogadro's number, which is about 6x10^23)
(so instead of saying there are about 6million,million,million,...,million,...,million atoms in a gram of hydrogen, we can say there are about 1 mole of atoms in a gram of hydrogen)
(don't ask me who chose to call it a mole!)

Just add these up for the number of atoms of each element in one molecule of each compound to get the molecular weights for each compound.
These are the number of grams of each compound that contain (approximately) 1 mole of molecules.

You can now replace all the weights in your problem with numbers of molecules.

Having done that, hopefully the rest of the question will become clearer
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Last edited by Woody; Sep 29th 2017 at 05:08 AM.
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Old Sep 29th 2017, 05:31 AM   #3
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Originally Posted by Woody View Post
Perhaps more of a Chemistry problem than Physics, but...

The key to this is converting the masses of each compound into an estimate of the number of atoms of each compound.
Any good depiction of the periodic table will include the atomic weights for each element.
These are the number of grams of each element that contain (approximately) 1 mole of atoms.
(A mole is just another name for Avogadro's number, which is about 6x10^23)
(so instead of saying there are about 6million,million,million,...,million,...,million atoms in a gram of hydrogen, we can say there are about 1 mole of atoms in a gram of hydrogen)
(don't ask me who chose to call it a mole!)

Just add these up for the number of atoms of each element in one molecule of each compound to get the molecular weights for each compound.
These are the number of grams of each compound that contain (approximately) 1 mole of molecules.

You can now replace all the weights in your problem with numbers of molecules.

Having done that, hopefully the rest of the question will become clearer
Thank you for your help, I solved it.
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