How do you calculate the mass in grams of stuff like molecules moles atom's and formula units?

If you have a formula for the compound, all you have to do is add up the atomic weights of the atoms. For example, let's say the molecule is "C2H4O2" (there are a number of molecules with this formula). You would calculate the molecular weight as follows:

2 carbon atoms (atomic weight of carbon is 12) = 24 (the units are grams/mole or grams/gram atom. Some people like to use "gram atom" for atoms while other people use "mole" for everything)
4 hydrogen atoms (atomic weight of carbon is 1) = 4
2 oxygen atoms (atomic weight of oxygen is 16) = 32
(Note that the atomic weights are approximate).

Molecular or formula weight of C2H4O2 is 24+4+32 = 60. (the units are grams/mole)

You now know that the molecular weight (or some people say the "formula weight") is 60 grams / mole. If you have 3 moles of the substance and wanted to know the number of grams you have, you would simply multiply 3 moles by 60 grams/mole:



If you had 40 grams ad wanted to know the number of moles, you simply divide by the molecular weight:



Note that you can use the dimensions of the formula weight (grams/mole) to figure out whether to multiply or divide. Remember your algebra. Moles times (grams/mole): "moles" will cancel and you'll have grams as in the first example. If you divide grams by (grams/mole) -- invert the divisor and multiply -- "grams" will cancel and you'll have moles.