Turning measured masses into moles to find the numbers in a balanced equation, the simplest formula of a compound, and its molecular formula.
What you will learn
- Use reacting masses to balance an equation
- Work out an empirical formula from masses or percentages
- Find a molecular formula from the empirical formula and Mr
Masses to an equation
If you know the mass of every substance in a reaction, you can find the balancing numbers without guessing.
- 4.8 g Mg reacts with 3.2 g O₂ to make 8.0 g MgO (Mg = 24, O = 16)
- moles: Mg 4.8 ÷ 24 = 0.20, O₂ 3.2 ÷ 32 = 0.10, MgO 8.0 ÷ 40 = 0.20
- divide by the smallest (0.10): 2 : 1 : 2
- 2Mg + O₂ → 2MgO
If dividing by the smallest leaves a half (1 : 1.5), multiply everything by 2.
The rest of this lesson and its 9 practice questions are locked.
In this lesson
- Masses to an equation
- Empirical formula
- Molecular formula
- Try it: Your first empirical formula
9 practice questions, marked as you go; about 20 minutes in all.
Key terms
Empirical formula, Water of crystallisation, Molecular formula
In this unit: Quantitative chemistry and the mole
Conservation of mass, relative formula mass, percentages and concentrations, the mole, reacting masses, empirical formulae, yield, atom economy, titrations and gas volumes.