**Calculating the percentage mass of an element in a compound**

- The percentage is the: Ar x number of atoms for that element in the formula / the Mr x 100
- E.g. MgCl2 (Mg Ar = 24 Cl Ar = 35.5)
- Mr = 24 +(35.5 x 2) = 95
- The percentage that is chlorine is:
- 5 x 2 / 95 x 100 = 74.7%

- E.g. MgCl2 (Mg Ar = 24 Cl Ar = 35.5)

You can use the percentage masses to work out the empirical formula of a compound

- Step 1: List the elements
- Step 2: Write their percentage or experimental masses
- Step 3: Divide each number by the Ar of that element
- Step 4: Turn the numbers you get into a ratio
- Step 5: in the simplest form that is the number of each element
- g Find the formula for iron oxide where 44.8g of iron react with 19.2g of oxygen (Ar = iron = 56 and O = 16)
- Step 1:
**Fe O** - Step 2: 8 19.2
- Step 3 8/56 = 0.8 19.2 / 16 = 1.2
- Step 4: x10 = 8 x10 = 12
- Step 5: 8:12
**2:3 so…**- This method is based on experiment to work out the formula for a compound
- It is the only way of doing it