molecular weight

The molecular weight of a substance is the sum of the atomic weights of the atoms in a molecule. The atomic weight is the average atomic mass for a naturally occurring element. This means that molecular weight is the average mass of a molecule of a substance. Molecular weight is expressed in atomic mass units. For example, we might want to find the molecular weight of a molecule of water. We have 2 atoms of H, with each hydrogen atom weighing 1 amu. We multiply 2 H atoms by 1 amu a piece to get 2 amu. We add the 16 amu from one O atom to the 2 amu from the oxygen to get a total of 18 amu for one molecule of water.

2 H * 1 amu = 2 amu
1 O * 16 amu = 16 amu

2 amu + 16 amu = 18 amu = molecular weight of water
H2O = 2 atoms of hydrogen and 1 molecule of oxygen

The formula weight of a substance is the sum of the atomic weights of all atoms in a formula unit of the compound. Formula weight doesn't depend on whether or not the substance is a molecule.For example, sodium chloride, which is NaCl, has a formula weight of 58.44 amu. This results from having 22.99 amu from Na and 35.45 amu from Cl. You would use the formula weight for substances which are not molecules, such as ionic compounds.

gram formula weight examples:

Find the gram formula weight of H2SO4
  1. 2H = 2 x 1 = 2
    1S = 1 x 32 = 32
    4O = 4 x 16 = 64
    98g/mole
Find the gram formula weight of Na2CO3 . 10 H2O
  1. 2Na = 2 x 23 = 46
    1C = 1 x 12 =12
    3O = 3 x 16 = 48
    10H2O = 10 x 18 = 180
    286g/mole


Sometimes we want to find out what the formula of a compound would be. To figure this out, we analyze the compound into amounts of the elements for a given amount of the compound. This is expressed as the percent composition which is the mass percentages of each different element in a compound. We must know the molecular weight of the compound in order to determine the molecular formula.

Say we have an element X in a compound. This element X is just part of the whole compound. We define the mass percentage of X as the parts of X per hundred parts of the total, by mass. That is:

Mass % X = (mass of X in the whole)/(mass of the whole) * 100%