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).