Empirical formulae from experiments, and concentration

You can find the formula of a compound like magnesium oxide by burning a known mass of metal. You also need to calculate concentration in g/dm3.

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Finding the formula of magnesium oxide

1. Weigh an empty crucible and lid. Add a coil of magnesium ribbon and weigh again.
2. Heat strongly, lifting the lid now and then to let oxygen in while stopping the white smoke (magnesium oxide) escaping.
3. When the magnesium stops glowing, cool and weigh. Heat again until the mass stops changing (heat to constant mass).
4. Mass of oxygen = final mass − mass of crucible and magnesium.

Empirical and molecular formulae

The empirical formula is the simplest whole-number ratio of atoms.
Method: divide each mass by its Ar, then divide by the smallest answer.
The molecular formula is the actual number of atoms in a molecule. Find how many empirical units fit into the Mr.
Example: empirical formula CH2 (mass 14), Mr = 56. 56 ÷ 14 = 4, so the molecular formula is C4H8.

Concentration in g/dm3

concentration (g/dm3) = mass of solute (g) ÷ volume of solution (dm3)
1 dm3 = 1000 cm3, so divide cm3 by 1000.
Example: 5 g dissolved to make 250 cm3: 5 ÷ 0.25 = 20 g/dm3.
Worked example

A crucible and lid weigh 25.00 g. With magnesium they weigh 25.24 g. After heating they weigh 25.40 g. Find the empirical formula. (Ar: Mg 24, O 16)

  1. Mass of Mg = 25.24 − 25.00 = 0.24 g; mass of O = 25.40 − 25.24 = 0.16 g
  2. Mg: 0.24 ÷ 24 = 0.01; O: 0.16 ÷ 16 = 0.01
  3. Ratio 1 : 1

Answer: MgO

Key idea

Burn magnesium in a crucible, lifting the lid, and heat to constant mass. Divide masses by Ar, then by the smallest, for the empirical formula. Molecular formula = empirical formula × (Mr ÷ empirical mass). Concentration = mass ÷ volume in dm3.

Key words
Concentration
The amount of solute in a given volume of solution, in g dm^{-3} or mol dm^{-3}.
Compound
A substance made of two or more elements chemically combined in fixed proportions.
Empirical formula
For each element: mass (or %) ÷ A_{r}; divide every answer by the smallest; round to a whole-number ratio (double if you get .5).
Molecular formula
The actual number of atoms of each element in one molecule.
Solute
The substance that dissolves in a solvent.
Solution
A mixture formed when a solute dissolves in a solvent.

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2 more questions

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  1. 6.0 g of salt is dissolved to make 150 cm3 of solution. Calculate the concentration in g/dm3.

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    Answer: 40 g/dm3

    150 cm3 = 0.15 dm3. 6.0 ÷ 0.15 = 40 g/dm3

  2. What mass of solute is in 500 cm3 of a solution of concentration 16 g/dm3?

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    Answer: 8 g

    500 cm3 = 0.5 dm3. mass = 16 × 0.5 = 8 g

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Sources and references

Checked by the GCSE-ready editorial team, 17 October 2026. Last updated 2 October 2026. Spotted a mistake? Tell us and we will fix it.

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