# 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 HClO_{4}

^{1}_{1.01}H

^{17}_{35.5}Cl

^{8}_{16}O

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 NH_{4}NO_{3}

^{8}_{16}O

^{1}_{1.01}H

^{7}_{14}N

% 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 NH_{4}NO_{3} 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.

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Date of last modification: Summer , 2019