Empirical formula lab – magnesium oxide, water of crystallisation and combustion analysis
ChemistryReactions & RedoxAges 16–17
Loading…
Sign in to playA virtual practical for finding formulas from experimental masses. Burn magnesium in a lidded crucible and heat to constant mass, drive the water of crystallisation out of a hydrated salt, or burn an organic compound and absorb the water and carbon dioxide. Convert masses to moles, find the simplest whole-number ratio for the empirical formula and use Mr from the mass spectrum for the molecular formula; the working shows how incomplete reaction, lost smoke or the wrong absorber order change the result.
Lesson: Empirical and molecular formulae from experimental data
What it shows
An empirical formula is the simplest whole-number ratio of atoms in a compound. It is found from masses: divide the mass of each element by its relative atomic mass to get amounts in moles, divide by the smallest amount and round to whole numbers. Burning magnesium shows that 0.24 g of Mg combines with 0.16 g of oxygen, giving MgO. Heating a hydrated salt gives the moles of water per mole of salt. In combustion analysis the carbon and hydrogen are found from the CO₂ and H₂O produced, and Mr from the mass spectrum turns the empirical formula into the molecular formula.
How to use
Choose an Experiment. For magnesium oxide or a hydrated salt, press Start heating, set the Lid or the Heating, then press Cool and weigh; repeat until the mass is constant. For combustion analysis, choose the Oxygen supply and Absorber order and press Burn the sample. Type your answer under Work out the formula and press Check. Show working displays the calculation from your own data and the errors in your run.
Parameters you can change
- Experiment Magnesium oxide (burning Mg in a crucible), Water of crystallisation (heating a hydrated salt), Combustion analysis (organic compound)
- Mass of magnesium 0.1–0.5 g
- Hydrated salt Copper(II) sulfate, Magnesium sulfate, Barium chloride
- Mass of hydrated salt 1–5 g
- Organic compound Ethanol, Propanone (acetone), Ethanoic acid, Ethyl ethanoate, Glucose, Benzene, Cyclohexane, Unknown compound X (random)
- Mass of sample burned 0.1–0.5 g
- Show the working straight away
Questions to explore
- Why must the crucible be heated, cooled and weighed again until its mass is constant?
- Why does losing white smoke make the formula of magnesium oxide seem to contain too little oxygen?
- Why must the water absorber come before the carbon dioxide absorber in combustion analysis?
