how do you convert from molecules to grams?

Answer 1

I suppose you know the molecular formula.
Look(and note) the atomoc wt of the constituent atoms from periodic table.
Add them up to get the molecular weight. For example, in H2O, molecular wt will be 2*atomoc wt of H + Atomic wt of O
now divide the molecular wt by avagadro number(6.023 x 10^23) to get the weight of one molecule in grams.
Happy calculating now!

Answer 2

To elaborate on Matahari's answer, there are 6.02 x 10^23 (Avogadro's number) molecules in a mole of any substance, so if you divide the number of molecules by Avogadro's number you get the number of moles of that substance. Multiply that by the atomic mass number for the substance, which is the number of grams per mole of that substance, and that yields the answer in grams.

Answer 3

you have to convert from molecules to moles using avogadro's number and then moles to grams using atomic/molecular mass