## Fundamentals E.21: Amount in moles vs Number of molecules vs Formula units

Molika Soben_3J
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Joined: Wed Sep 21, 2016 3:00 pm

### Fundamentals E.21: Amount in moles vs Number of molecules vs Formula units

I was going through the Fundamentals section and formula units kept on popping up. For example, in problem E.21, it asks to calculate the amount in moles and the number of molecules and formula units in a 10.0 g of alumina. Will someone explain to me the difference between number of moles, molecules, and formula units? I'm a little confused about the concept of formula units.
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Noor_Chahal_3G
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Joined: Wed Sep 21, 2016 2:56 pm

### Re: Fundamentals E.21: Amount in moles vs Number of molecules vs Formula units

In terms of calculation, you would treat formula units and molecules in the same way. If you're trying to find how many formula units/molecules are present, you would multiply the number of moles of the substance by 6.022x1023 in both cases.

To get more technical: a molecule refers to a group of 2 or more elements that are covalently bonded (nonmetal-nonmetal)
A formula unit refers to an individual ionic compound (metal-nonmetal)

For example, in one mole of H2O, there are 6.022x1023 molecules of H2O
in one mole of CaCl2 there are 6.022x1023 formula units of CaCl2

Annie Chang 3G
Posts: 16
Joined: Wed Sep 21, 2016 2:59 pm

### Re: Fundamentals E.21: Amount in moles vs Number of molecules vs Formula units

The number of moles measures the number of particles in a specific substance. To convert between the mass of the substance to the number of moles in that substance, the molar mass can be used. For example, in part a of the problem, to convert from 10.0g of alumina to moles of alumina, you would use the molar mass of Al2O3, 101.96 g/mol.
10.0g Al2O3 * (1 mol/ 101.96g )= 0.0981 mol Al2O3

Avogadro's number, 6.02x10^23, represents the number of atoms, molecules, or ions in one mole of a substance. So, to find the number of molecules in Al2O3, you would multiple the moles of Al2O3 with 6.02 x l0^23 molecules per mole.
0.0981 mol Al2O3 * 6.02 x l0^23 molecules/mol= 5.91x10^22 AL2O3 molecules.

I believe the formula unit is the general term for any combination of atoms. So, like Noor Chahal said in the previous post, Avogadro's number is used to find the formula unit like for finding the number of atoms and molecule.

Katelyn Li 2J
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Joined: Wed Sep 21, 2016 3:00 pm

### Re: Fundamentals E.21: Amount in moles vs Number of molecules vs Formula units

I have another question about this problem, specifically part b. I worked it through and thought it required 4 sig figs, but when I checked the solutions manual, it was rounded to 3. Can someone explain why the answer is given with 3 sig figs?

Pauline Tze 3B
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### Re: Fundamentals E.21: Amount in moles vs Number of molecules vs Formula units

The sig fig rule is that non-zero digits are always significant, so your answer is correct in having 4 sig figs.
The 3 sig fig answer is probably a typo in the solutions manual (which may be because all the other parts in the question have 3 sig figs).

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