STUDENT SPACE · TOPIC 5

Empirical and molecular formulae

Deduce empirical formulae from composition and molecular formulae from relative molecular mass.

The essentials

The empirical formula is the simplest whole-number ratio of atoms or ions in a compound.
The molecular formula gives the actual numbers of atoms of each element in one molecule.
When magnesium reacts with oxygen, the mass increase gives the mass of oxygen combined.
2.4 g Mg + 1.6 g O: moles = 0.10 : 0.10, so empirical formula = MgO.

Try each question before opening its hint or answer. Use paper for calculations and diagrams.

Can empirical formula CH₂O alone determine the molecular formula? Explain.

Need a hint for Question 1?

Use the relationships above and check particle ratios and units.

Check Question 1

No. The relative molecular mass (or equivalent information) is also needed.

Find the empirical formula of a compound containing 3.2 g S and 4.8 g O. Aᵣ: S = 32, O = 16.

Need a hint for Question 2?

Convert masses to moles and divide by the smaller amount.

Check Question 2

Moles S = 0.10, O = 0.30; ratio 1:3 → SO₃.

A compound has empirical formula CH₂ and Mᵣ = 56. Find its molecular formula. Aᵣ: C = 12, H = 1.

Need a hint for Question 3?

Divide molecular mass by empirical-formula mass.

Check Question 3

Empirical formula mass = 14; multiplier = 56/14 = 4 → C₄H₈.

Use Lesson 5 in your worksheet for written practice. Your teacher will discuss those answers in class.