The molecular mass of a molecule is the total mass of all the atoms making up the molecule. This example problem illustrates how to find the molecular mass of a compound or molecule.
Find the molecular mass of table sugar (sucrose), which has a molecular formula C12H22O11.
To find the molecular mass, add the atomic masses of all of the atoms in the molecule. Find the atomic mass for each element by using the mass given in the Periodic Table. Multiply the subscript (number of atoms) times the atomic mass of that element and add the masses of all of the elements in the molecule to get the molecular mass. If we round off the atomic masses to four significant figures, we get:
molecular mass C12H22O11 = 12(mass of C) + 22(mass of H) + 11(mass of O)
molecular mass C12H22O11 = 12(12.01) + 22(1.008) + 11(16.00)
molecular mass C12H22O11 = = 342.30
Note that a sugar molecule is about 19 times heavier than a water molecule!