# This whole unit thing of mols is somewhat tricky and interesting. if

1. Feb 3, 2012

### LT72884

This whole unit thing of mols is somewhat tricky and interesting. if one mol of Na is 23g, then wouldnt 100.3g of NaCl be one Mol as well. I am under the impression from my lecture notes in class that the grams of a substance is the same a s a mol. I was asked a school of thought question by the profesor. How many mols is in 100.3g of NaCl. I said one because the notes stated that the grams of the substance is also equal to one mol. But as i write this i am puttign things together.

is it only one mol IF the grams of whatever happen to be equal to the amu. so NaCl has a molar mass of 58. if the question was asked, if you had 58g of NaCl, then that would be ONE mol right. it couldnt be the 100.3g is one mol because the 100.3g is not equal to the amu of NaCl

thanks guys.

2. Feb 3, 2012

### ardie

Re: mols

not quite so. one mol of any substance, is the atomic mass of its constituents, times the Avogardo's constant (a very large number). the atomic mass is in terms of the atomic mass unit, where each nucleon weights roughly 1 units. the avogardo's constant then can connect your nucleon mass to grams, because one mol of anything, weights exactly the same as the atomic mass number (or the number of nucleons) of that thing in grams. 100 grams of NaCl is therefore 100/58 or roughly 2 mols.

3. Feb 3, 2012

### nasu

Re: mols

You mean the molar mass is what follows after the bold part, I suppose.

4. Feb 3, 2012

### ardie

Re: mols

you dont say one molar mass of chlorine do you, you say one mol of chlorine, and what you mean is an avogardo's number of them, which weigh one avogadro's number times their atomic mass times the mass of nucleons in grams.

Last edited: Feb 3, 2012
5. Feb 5, 2012

### nasu

Re: mols

"One mol" and the "mass of a mol" are two different things.
Multiplying atomic mass of the constituents by Avogrado's number you obtain the mass of a mol of that substance and this is called molar mass.
The result is the mass of the mol and not the mol itself.
A mol is defined without any reference to the mass of the constituents. It's just Avogrado number of particles.
More general, the mass of an object is not the same thing as the object itself.