# Formula Weight and % Composition from Formula

## Formula Weight

### Formula weight of a molecule

The summation of the atomic weights of all atoms In the chemical formula of the molecule.
Note: Formula weight calculated by this way is also known as "Molecular Formula Weight" or "Molecular Weight"

Example: Calculate the formula weight of HClO4
11.01H

1735.5Cl

816O

Solution: Molecular Weight = (1 × 1) + (1 × 35.5) + (4 × 16) = 100.5 amu

### Exercise: Calculate the formula weight of the following molecules

Exercise on Calculating the Formula Weight of Molecules

Check your answers here: Solution to the Exercise on Calculating the Formula Weight of Molecules

## % Composition from Formula

The percentage (%) of each element in a molecule can be calculated if the molecular formula of the molecule is given as follows:
% Element =
(((# of atoms of the element) × (atomic weight of the element)) / (Formula weight of the compound))) × 100

### Example: Calculate the % N in NH4NO3

816O

11.01H

714N

% N = (((# of atoms of the element) × (atomic weight of the element))/ (Formula weight of the compound))) × 100 =
= (2 × 14) / ((1 × 14) + (4 × 1) + (1 × 14) + (3 × 16)) = 35; which means that NH4NO3 contains 35% of N.

Hence, if the molecular formula of a molecule is known, we should be able to calculate the % of any of its elements, which means the % composition of any element in this molecule.