1. Percent Composition & Empirical and Molecular Formulas

2. Percent Composition
Gives the percent, by mass, of each element in a compound.
1st – Determine the mass of each element. If there are more than one atom of the element in the compound, be sure to multiply by the number of atoms.
2nd – Calculate the sum of all the masses of the elements to find the molar mass of the compound.
3rd – Determine the percent by mass of each element by taking their total mass, divided by the molar mass, and multiplying by 100.

3. Examples
H2O %H - %O -
C6H12O6
%C –
%H –
%O –

4. Try this one on your own
Calculate the percent composition of aspartame, C14H18N2O5.

5. Empirical Formula
Gives the lowest whole-number ratio of the atoms of the elements in a compound.
This may, or may not, be the same as the molecular formula.
Hydrogen Peroxide
Molecular Formula –
H2O2
Empirical Formula – HO
Glucose
Molecular Formula –
C6H12O6
Empirical Formula –
CH2O
Tetranitrogen decoxide
Molecular Formula –
N4O10
Empirical Formula –
N2O5

6. Calculating Empirical Formulas
Step 1 – Treat compound as 100g. This will make all percentages become grams. Step 2 – Convert each mass to moles. Step 3 – Divide each of the moles by the smallest number. Step 4 – If this creates a fraction of a mole, for any element, multiply all moles by a number to convert this to a whole number. Ex. If you get N – 1 and O – 2.5 Multiply by 2, and get N – 2, and O – 5.

7. Example
Determine the empirical formula of a molecule containing the following percent composition. 65.5% C 5.5% H 29.0% O
If the molar mass of the compound is 110 g/mol, what is the molecular formula?

8. Example
Determine the empirical formula of a molecule containing the following percent composition. 18.7% Li 16.3% C 65% O
If the molar mass of the compound is 73.8 g/mol, what is the molecular formula?