1.33 calculate empirical and molecular formulae from experimental data
Calculating Molecular Formula
1 | Write the element symbols | S | O |
2 | Write the masses or the % of the elements | 3.2 g | 6.4 g – 3.2 g = 3.2 g |
3 | Write the Ar values of each element | 32 | 16 |
4 |
Divide masses by At
| 3.2 ÷ 32 = 0.1 | 3.2 ÷ 16 = 0.2 |
5 | Divide by the smallest number | 0.1 ÷ 0.1 = 1 | 0.2 ÷ 0.1 = 2 |
6 | Write the formula | SO2 |
Calculating Molecular Formula
From the empirical formula, you can work out the molecular formula from the relative formula mass (Mr) of the compound.
- Add up the atomic masses of the atoms in the empirical formula.
For example, the empirical formula of a hydrocarbon is CH2 and its Mr is 42.
- The mass of the atoms in the empirical formula is 14
- 42 ÷ 14 = 3
- so you need to multiply the numbers in the empirical formula by 3
The molecular formula of the hydrocarbon is therefore C3H6.
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