All subjects · Chemistry · WCH11 (Unit 1)

Empirical and molecular formulae

An empirical formula is the simplest whole-number ratio of atoms in a compound. You can find it from experimental masses or percentages, then use the molar mass to get the molecular formula.

The method

1. Write the mass (or percentage) of each element.

2. Divide each by its relative atomic mass to get moles.

3. Divide every answer by the smallest.

4. If a value ends in about .5, .33 or .25, multiply all of them to get whole numbers.

Molecular formula

Work out the mass of one empirical unit. Divide the molar mass by it to find how many units there are. For CH2O (30.0) with Mr = 180.0: 180.0 / 30.0 = 6, so C6H12O6.

Water of crystallisation

Hydrated salts such as CuSO4.xH2O lose water on heating. Find the mass of water lost and the mass of anhydrous salt, convert both to moles, and the ratio gives x. Heat to constant mass to be sure all the water has gone.

Worked example

A compound contains 40.0% C, 6.7% H and 53.3% O by mass, and has Mr = 180. Find its molecular formula.

  1. Moles: C 40.0/12.0 = 3.33; H 6.7/1.0 = 6.7; O 53.3/16.0 = 3.33
  2. Divide by 3.33: C 1, H 2, O 1, so CH2O
  3. Empirical mass 30.0; 180 / 30.0 = 6

Answer: C6H12O6

Key idea: Mass to moles, divide by the smallest, make whole numbers; molecular formula = (Mr / empirical mass) x empirical formula.

Check yourself

  1. A compound contains 2.4 g of carbon and 0.6 g of hydrogen. What is its empirical formula? (C = 12.0, H = 1.0)
    • CH4
    • C2H5
    • CH3
    • CH2
    Show answer

    CH3. C: 2.4/12.0 = 0.20; H: 0.6/1.0 = 0.60; ratio 1 : 3.

  2. A hydrocarbon has empirical formula CH2 and Mr = 70.0. How many carbon atoms are in one molecule? (C = 12.0, H = 1.0)
    Show answer

    5. CH2 has mass 14.0; 70.0 / 14.0 = 5, so the formula is C5H10.

  3. 2.50 g of hydrated copper(II) sulfate, CuSO4.xH2O, is heated to constant mass, leaving 1.60 g of anhydrous CuSO4. Find x. (Cu = 63.5, S = 32.1, O = 16.0, H = 1.0)
    Show answer

    5. Water = 0.90 g = 0.0500 mol. CuSO4 = 1.60 / 159.6 = 0.0100 mol. Ratio 0.0500 : 0.0100 = 5, so x = 5.

  4. Mole ratio calculations give Fe 1 : O 1.5. What is the empirical formula?
    • FeO
    • Fe2O3
    • FeO2
    • Fe3O2
    Show answer

    Fe2O3. Multiply both by 2 to get whole numbers: 2 : 3.

  5. Why is a hydrated salt heated to constant mass?
    • To make the salt react with oxygen
    • To cool it down evenly
    • To increase the mass of the salt
    • To make sure all the water of crystallisation has been removed
    Show answer

    To make sure all the water of crystallisation has been removed. If the mass stops changing, no more water is being lost.

  6. Find the percentage by mass of nitrogen in NH4NO3. (N = 14.0, H = 1.0, O = 16.0)
    Show answer

    35.0 %. Mr = 80.0; mass of N = 28.0; 28.0 / 80.0 x 100 = 35.0%.

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Last updated 7 November 2026. Spotted a mistake? Tell us.