To convert from molecules to grams, it is necessary to first convert the number of molecules of a substance by dividing by Avogadro's number to find the number of moles, and then multiply the number of moles by the molar mass of this substance. Avogadro's number is given as 6.022 x 10^23.
Avogadro's number is a constant that represent the number of molecules or atoms per mole of any given substance. To work out a calculation, student must also know that the molar mass of the substance, which can be found from its chemical formula and is given in grams per mole. Letting N be the number of molecules or moles of the substance, the formula to convert from molecules to grams is then given as (N number of molecules) /(6.022 x 10^23) x (substance's molar mass in grams/mol).