Determine the molecular molar mass of the compound. For instance, if the compound is water (H2O), its molecular molar mass will be 2 x (atomic molar mass of hydrogen) + molar mass of oxygen. Refer to the periodic table for determining the atomic molar masses of all the known elements. Use this information to get the molecular molar mass of the compound. Let's assume that the molecular molar mass of the given compound comes to 60 grams.
Write down the following equation on a piece of paper: 1 molar mass of the compound has 6.022 x 10^23 molecules. From Step 1, 1 molar mass of the compound is equal to 60 grams, so 60 grams of the compound have 6.022 x 10^23 molecules. From this equation, we can deduce that 1 gram of the compound will have (6.022 x 10^23)/60 molecules.
Determine the number of molecules in the given mass of the compound. Let's assume that you have a sample of the compound which weighs 120 grams. If 1 gram of the compound has (6.022 x 10^23)/60 molecules, then 120 grams of the compound will have [(6.022 x 10^23)/60] x 120, which is equal to 12.022 x 10^23 molecules.