Empirical formulae
The empirical formula of a compoundA substance formed by the chemical union of two or more elements. is the simplest whole number ratio of atomAll elements are made of atoms. An atom consists of a nucleus containing protons and neutrons, surrounded by electrons. of each elementA substance made of one type of atom only. in the compound. It is determined using data from experiments and therefore empirical.
For example, the molecular formula of glucose is C6H12O6 but the empirical formula is CH2O. This is because we can divide each number in C6H12O6 by 6 to make a simpler whole number ratio.
On the other hand, a compound which has the empirical formula of CH2 could have a molecular formula of C2H4, C3H6, C4H8 or even C13H26.
You can use information about reacting massThe amount of matter an object contains. Mass is measured in kilograms (kg) or grams (g). to calculate the formula of a compound.
Example
3.2 g of sulfur reacts with oxygen to produce 6.4 g of sulfur oxide. What is the formula of the oxide?
For your calculation you鈥檒l need to use the fact that the Ar (relative atomic mass) of sulfur, S = 32, and the Ar of oxygen, O = 16.
| Step | Action | Result | |
| One | Write the element symbols | S | O |
| Two | Write the masses | 3.2 g | 6.4 g 鈥 3.2 g = 3.2 g |
| Three | Write the Ar values | 32 | 16 |
| Four | Divide masses by Ar | 3.2 梅 32 = 0.1 | 3.2 梅 16 = 0.2 |
| Five | Divide by the smallest number | 0.1 梅 0.1 = 1 | 0.2 梅 0.1 = 2 |
| Six | Write the formula | SO2 |
| Step | One |
|---|---|
| Action | Write the element symbols |
| Result | S |
| O |
| Step | Two |
|---|---|
| Action | Write the masses |
| Result | 3.2 g |
| 6.4 g 鈥 3.2 g = 3.2 g |
| Step | Three |
|---|---|
| Action | Write the Ar values |
| Result | 32 |
| 16 |
| Step | Four |
|---|---|
| Action | Divide masses by Ar |
| Result | 3.2 梅 32 = 0.1 |
| 3.2 梅 16 = 0.2 |
| Step | Five |
|---|---|
| Action | Divide by the smallest number |
| Result | 0.1 梅 0.1 = 1 |
| 0.2 梅 0.1 = 2 |
| Step | Six |
|---|---|
| Action | Write the formula |
| Result | SO2 |
The action at step five usually gives you the simplest whole number ratio at once. Sometimes it does not, so you might get 1 and 1.5. In this example, you would multiply both numbers by 2, giving 2 and 3 (instead of rounding 1.5 up to 2).
Converting the empirical formula to a molecular formula
From the empirical formula, you can work out the molecular formula if you know the relative formula massThe relative formula mass (Mr) of a compound is calculated by adding together the relative atomic masses (Ar) of the atoms present in the compound. (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 梅 14 = 3
- so you need to multiply the numbers in the empirical formula by 3
The molecular formula of the hydrocarbon is therefore C3H6.