A molecule is the smallest unit of a substance that still retains the characteristics of that substance. It is too small to see with the naked eye or to weigh on a mass balance. Yet scientists often need to know its mass. To calculate this, they use a unit called the mole and a measurement called the atomic weight. A mole is a quantity equal to Avogadro's number, which is roughly 6.022 x 10^23. The atomic weight is the weight of a single atom of an element in atomic mass units. One mole of an element weighs the same amount in grams as the element's atomic weight. Knowing this, scientists can derive the mass of any molecule from its chemical formula and the atomic weights of its elements as described in the Periodic Table.
Identify the molecule's chemical formula.
For example, to calculate the mass of a molecule of water, you must first recognize its formula as H20.
Find the atomic weight of each atom in the molecule. You can find this information on the periodic table; it is usually a decimal number written above or below the chemical symbol. The atomic mass is also equal to the mass per mole, or molar mass, of that element.
For example, the atomic mass of hydrogen is 1.0079 and that of oxygen is 15.999. One mole of each element weighs the same amount in grams.
Add the weights of each element in the molecule to calculate the total molar mass.
For example, the molar mass of water is equal to 1.0079 + 1.0079 + 15.999, or 18.0148 grams per mole (g/mol).
Divide the mass per mole by the number of molecules per mole, or 6.022 x 10^23. The result is the mass of one molecule.
For example, 18.0148 / 6.022 x 10^23 = 2.991 x 10^23. Therefore, one molecule of water weighs 2.991 x 10^23 grams.