Gram Formula Mass

This is a calculation of the mass of one mole of a compound. It is simply the sum of the masses of all the atoms in the substance. For example, a mole of LiCl, lithium chloride is 6.941+35.45. These are the masses on the periodic table for Li and Cl. So the answer is 42.391g. So if I weighed 42.391g exactly of LiCl, I would have 1 mole of the compound, or 6.02 x 10²³ molecules of LiCl.

Its another cool thing about the Periodic Table.

The atomic mass numbers can be used two ways!

We can use them to determine the mass of an atom in amu's (atomic mass units), and use it to determine the number of neutrons.

Or, we can change the unit to grams, and use it to determine the mass of one mole (6.02 x 10²³) atoms of any element.

Also, one mole of lithium atoms will have a mass of 6.941 grams.

Okay, so you got that. Now, what about magnesium bromide? MgBr₂? Well, you have one magnesium and 2 bromines. So The gram formula mass is 24.31 plus 2 times 79.90 because there are 2 bromines. So the gram formula mass is 184.11g.

Well, how many of each atom does it have?

2Fe, 3S, 12O

So the atomic mass of Fe is 55.85, atomic mass of S is 32.07 and the atomic mass of O is 16.00

So 2 x 55.85 = 111.70

+3 x 32.07 = 96.21

+12 x 16.00 = 192.00

The gram formula mass = 399.91g. If I have 399.91g of iron(III) sulfate, I have 6.02 x 10²³ atoms, or one mole.