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Empirical FormulaOxford AQA IGCSE Chemistry: Revision notes

Section 1

What is an empirical formula?

The empirical formula of a compound is the simplest whole-number ratio of atoms of each element present.

This is different from the molecular formula, which shows the actual number of atoms of each element in one molecule.

For example, glucose has the molecular formula C6H12O6. Dividing every number by the highest common factor (6) gives the empirical formula CH2O.

For some compounds the empirical and molecular formulae are the same (e.g. water, H2O) — this happens when the ratio is already in its simplest form.

Key termsempirical formulamolecular formula

Section 2

How do you calculate an empirical formula from masses?

To find an empirical formula from the masses of elements in a compound:

  1. Divide each mass by the element's relative atomic mass (Ar) to get the number of moles of atoms
  2. Divide every answer by the smallest of the mole values
  3. If the results are not whole numbers, multiply all of them by the same small number (e.g. 2) until they are
  4. Write the whole-number ratio as the formula
Key termsmole ratio
Example

A compound contains 2.4 g carbon and 0.6 g hydrogen (Ar: C = 12, H = 1). Moles: C = 2.4/12 = 0.2, H = 0.6/1 = 0.6. Divide by smallest (0.2): C = 1, H = 3. Empirical formula = CH3.

Section 3

How do you calculate an empirical formula from percentages?

If you are given percentage composition instead of masses, treat each percentage as if it were a mass in grams (because the percentages are out of 100).

Then follow exactly the same method: divide each 'mass' by its Ar, divide by the smallest value, and scale up to whole numbers if needed.

Exam tip

Examiners award marks for showing each step (moles calculated, division by smallest, final ratio) — always show your working even if the final answer is wrong.

Section 4

How do you find a molecular formula from an empirical formula?

The molecular formula is always a whole-number multiple of the empirical formula.

  1. Calculate the relative formula mass (Mr) of the empirical formula
  2. Divide the given Mr of the actual molecule by this value to find the multiplier, n
  3. Multiply every number of atoms in the empirical formula by n
Example

Empirical formula CH2O has a mass of 30. The compound's actual Mr is 180. n = 180/30 = 6. Molecular formula = C6H12O6.

Must Know

  • Empirical formula = simplest whole-number ratio of atoms
  • Molecular formula = actual number of atoms in a molecule
  • To find empirical formula: mass (or %) ÷ Ar → divide by smallest → scale to whole numbers
  • Percentages can be treated as masses out of 100 g
  • Molecular formula = empirical formula × n, where n = Mr(molecular) ÷ Mr(empirical)
  • Always show every step of working for full marks

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