[solomon_pup]

I apologize for all of the questions but this will be my last one.  I just have a hard time desiphering these questions:

One mole of a pure substance is:
a) an Avogadro's number of the parrticles that make up the chemical formula.
b)  the amount of that substance present when the formula weight of substance is weighed out in grams.
c)  6.02X10²³ molecules or chemical formulas of the substance.
d)  all of the above

I originally put d but now I just want to put b.  I get confused sometimes with the difference between molecules/atoms/elements, etc.  the C kind of confused.  I know for example, H₂O - 2 hydrogen and 1 oxygen, which would be 17.  So one mole of H₂O would be 17 grams.  So that means 6.02 X 10²³ molecules of H₂O = 17 grams, correct?  In that case I guess I should stick to my original answer of all of the above.

OI!

[Albert]

I think this will help you to understand: http://en.wikipedia.org/wiki/Mole_(unit)

[Yggdrasil]

D is the correct answer and here's an explanation:

Lets use b as a starting point, since you are sure that this is true.  Now, what makes a mole so special that it is the exact amount of substance where the formula weight equals the mass in grams?  First lets consider what formula weight is.

Atoms consist of protons, neutrons and electrons.  Since the mass of electrons is negligible compared to the mass of protons and neutrons, we can ignore these for our purposes.  Protons and neutrons have a mass of one atomic mass unit (amu or dalton).  For simplicity's sake, lets consider helium-4, a single isotope.  One atom of ⁴He consists of two protons and two neutrons.  Therefore each atom has a mass of 4 amu and thus the formula weight of ⁴He is 4.  From definition b, we know that one mole of ⁴He would weigh 4 g.  Now how do we find how many He atoms is in 4g?

Well, we need a conversion factor (lets call it N) so that:

4 g x (N amu / g) = 4 amu

In other words, we need to find the weight of 1 gram in amu.  It turns out that:

6.02x10²³amu = 1g

But, since protons and neutrons have a mass of 1 amu, you can say that:

6.02x10²³ protons = 1g
and
6.02x10²³ neutrons = 1g

This number, like many other constants, is so important that it has been given a name, Avogadro's number (note: the definition of Avogadro's number is slightly different, but it is basically the number of protons in a gram).

With this in mind, we can now see that a and c are equivalent since Avogadro's number = 6.02x10²³.  And we can also see that a and c are true.  Why?  Well, the formula weight is basically just the sum of the number of protons and neutrons in a molecule/formula unit of a substance.*    So, if you have a molecule of water, it has ten protons and eight neutrons.  Therefore Avogadro's number of water molecules has 10x(6.02x10²³) protons and 18x(6.02x10²³) neutrons.  Therefore the weight of protons in 6.02x10²³ molecules of water is 10g and the weight of neutrons in this amount of water is 8g, giving a total mass of 18g.

*Basically for covalent substances you have molecules, but for ionic substances, like NaCl, molecules of NaCl don't exist since Na and Cl ions are bound in a repeating lattice.  So, chemists call the simplest ratio of Na to Cl ions in the lattice a "formula unit."  Basically, it's the same concept as a molecule of NaCl, but chemists call it a "formula unit" because it isn't correct to say that there are molecules of NaCl.