text: how many molecules are in 1.00g of iodomethane? how many molecules are in 1.00g of iodomethane? what is the relative mass of methane? how many moles of methane are there in4g of pure methane? i'm not going to give you the answers, because you can really work them out for yourself.  you need to work out the molecular weight, which is just the sum of the atomic weights in the appropriate proportions. say that works out to be 100 for a particular molecule: that means that 100g of the substance contains 1 mole (6.022 x 10^23) molecules - so 1 gram contains one hundredth of that, or 6.022 x 10^21 molecules.  apply that to each of the molecules in your questions, and you'll be home and dry.