Cambridge IGCSE Chemistry · 0620

Chapter 3: Stoichiometry — Part 5

Topic 3.3c · Empirical and molecular formulae calculations

01 Empirical formula calculations

From percentage composition

Treat the percentages as masses in 100 g of compound. Divide each mass by A_r to get moles of each element. Divide all values by the smallest to get a ratio. If ratios are not whole numbers, multiply all by 2 or 3 to obtain whole numbers.

From mass data

Same method: divide each element's mass by A_r, find the simplest ratio, then write the empirical formula.

Table method

List each element with columns for mass, ÷ A_r, and ratio. The empirical formula uses the ratio as subscripts.

02 Molecular formula calculations

Relationship between formulae

Molecular formula = (empirical formula)_n, where n is a whole number.

Finding n

Calculate the M_r of the empirical formula. Then n = M_r (molecular) ÷ M_r (empirical).

Final step

Multiply each subscript in the empirical formula by n to obtain the molecular formula. Example: empirical CH₂O, M_r = 60, empirical M_r = 30 ? n = 2 ? molecular formula C₂H₄O₂.