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Form 4 · Chapter 3

Chemical Formula

Distinguish empirical from molecular formulae and learn to determine them from masses, percentages and ionic charges.

Empirical and molecular formulae

A chemical formula shows the elements in a compound and their ratio. The empirical formula gives the simplest whole-number ratio of atoms, while the molecular formula gives the actual number of atoms in one molecule. For example, hydrogen peroxide has empirical formula HO but molecular formula H2O2; glucose is CH2O (empirical) and C6H12O6 (molecular).

Formulae of ionic compounds

For ionic compounds, balance the total positive and negative charges so the compound is neutral. Cross-multiply the charges to find the subscripts.

IonFormula
SodiumNa+
CalciumCa2+
ChlorideCl−
SulfateSO42−

So calcium chloride is CaCl2 and aluminium oxide is Al2O3.

Worked example

A compound contains 40% calcium, 12% carbon and 48% oxygen by mass. Find the empirical formula. (Ar: Ca = 40, C = 12, O = 16)
Moles: Ca = 40/40 = 1; C = 12/12 = 1; O = 48/16 = 3.
Ratio Ca : C : O = 1 : 1 : 3, so the empirical formula is CaCO3.

Remember

  • Empirical = simplest ratio; molecular = actual number.
  • To find the empirical formula: divide mass (or %) by Ar, then simplify.
  • Ionic formulae must balance total charge to zero.

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