Empirical Formula Virtual Lab: Formula of Magnesium Oxide

This empirical formula virtual lab runs the classic magnesium oxide practical from GCSE, IGCSE, A-level and IB Chemistry. Students weigh a crucible and lid, add magnesium ribbon, heat it while lifting the lid now and then, and repeat heating, cooling and weighing until the mass is constant. They turn the masses of magnesium and oxygen into moles and find the simplest whole-number ratio. The simulation also shows what goes wrong when the lid stays on or comes off, and a second experiment finds the water of crystallization in hydrated copper(II) sulfate.

Curriculum links

• AQA GCSE Chemistry 4.3.1.3 (mass changes when a reactant is a gas) and 4.3.2 (amounts of substance, higher tier).
• AQA A-level Chemistry 3.1.2.4 (empirical and molecular formula).
• Cambridge IGCSE Chemistry 0620, topic 3.3 (the mole: empirical formulae and water of crystallization).
• IB Chemistry Structure 1.4 (counting particles by mass) and AP Chemistry topic 1.3 (elemental composition of pure substances).

Simulic is not affiliated with or endorsed by AQA, Cambridge, the IB, the College Board or any exam board.

Before the lab (5 min)

Ask students to commit to a prediction, on paper or as question 1 of the class link:

"You heat 0.24 g of magnesium ribbon in a crucible until it has all reacted. Does the mass of the crucible and its contents go up, go down or stay the same?"

Many students say it goes down, because "the magnesium burns away". Don't correct them yet.

Method in the simulation

1. Keep Experiment on "Magnesium oxide (burning Mg)" and Mass of magnesium at 0.24 g. The table already shows the empty crucible and lid, and the crucible, lid and magnesium.
2. Set Lid to "lifted slightly" and press Start heating. The magnesium glows while it reacts.
3. When it no longer glows, press Cool and weigh. The new mass appears in the results table.
4. Press Start heating again for a short time, then Cool and weigh. Repeat until the status line says "Constant mass": the last two weighings agree to within 0.002 g.
5. Calculate the mass of magnesium and the mass of oxygen, then the amount of each in moles (Ar: Mg = 24.31, O = 16.00). Type your formula under Work out the formula and press Check.
6. Press Restart and repeat with Lid "removed", then with Lid "on". Compare the ratios.
7. Optional: repeat steps 2–4 with 0.12 g and 0.36 g of magnesium and plot the mass of oxygen against the mass of magnesium.

Mass of Mg (g)	Lid	Crucible + lid (g)	Crucible + lid + Mg (g)	Constant final mass (g)	Mass of O (g)	mol O ÷ mol Mg
0.24	lifted slightly
0.24	removed
0.24	on

Expected results

Values from the simulation. The balance reads to 0.001 g and its last digit wobbles by ±0.001 g, so groups differ slightly.

Lid	Mass of Mg (g)	Mass of O (g)	mol O ÷ mol Mg	What the working says
lifted slightly	0.241	0.154	0.97	MgO, no procedural errors
removed	0.239	0.113	0.72	smoke escaped, final mass too low
on (5 min, then 6.6 min)	0.242	0.080, then 0.094	0.50, then 0.59	not all the Mg reacted; no constant mass

• With the lid lifted slightly, the mass of oxygen is 0.152–0.156 g for 0.24 g of magnesium, and the ratio is close to 1 : 1. The percentage yield of MgO is about 99%.
• The mass of oxygen is proportional to the mass of magnesium: about 0.078 g for 0.12 g and 0.230 g for 0.36 g.
• Water of crystallization: 2.50 g of hydrated copper(II) sulfate, heated gently and cooled in a desiccator, loses 0.901 g of water (36.0%). That gives x = 4.99, so the formula is CuSO₄·5H₂O.

Questions for students

1. (Prediction, asked again after the lab) How does the mass of the crucible and contents change when magnesium is heated in it?
2. A class uses 0.12 g, 0.24 g and 0.36 g of magnesium. Which is the dependent variable?
3. With 0.24 g of magnesium and the lid lifted slightly, what mass of oxygen combines?
4. From your masses, what is the amount of oxygen divided by the amount of magnesium?
5. With the lid removed, the ratio Mg : O is about 1 : 0.72. Why?

Answers for teachers: (1) It increases, by about 0.154 g, because oxygen from the air combines with the magnesium. (2) The mass of oxygen that combines (the mass of magnesium is the independent variable). (3) About 0.154 g (accept 0.150 to 0.160 g). (4) About 0.97 (accept 0.94 to 1.03), so the formula is MgO. (5) White magnesium oxide smoke escaped, so the final mass, and so the mass of oxygen, is too low.

Common misconceptions

• "Burning makes things lighter." Magnesium gains mass because it combines with oxygen. Mass is conserved, but the oxygen comes from air that was never on the balance.
• "Keep the lid on so nothing escapes." With the lid on, too little air gets in and the magnesium only half reacts. The lid must be lifted from time to time.
• "A ratio of 0.97 means the formula is Mg₃₃O₃₂." Experimental ratios are never exact. Round to the simplest whole-number ratio that fits within the experimental error.

Extension

• Switch Experiment to "Water of crystallisation", choose copper(II) sulfate and find x. Then try strong heating, or untick Cool in a desiccator, and read the error the working reports.
• Combustion analysis (A-level and IB): burn ethyl ethanoate, find the masses of C, H and O from the CO₂ and H₂O absorbed, and use the molecular ion peak to get the molecular formula.

FAQ

Why does the ratio come out a little below 1?

With the lid lifted slightly, about 1% of the magnesium oxide escapes as smoke, as in a real crucible. The ratio is still close enough to 1 : 1 to give MgO.

Does the simulation include magnesium nitride?

No. The small amount of nitride that forms in air is ignored, so the only errors come from the lid, incomplete heating and the balance.

How long does one run take?

About 2 minutes: 25 seconds of heating shows as 5 minutes on the clock. That leaves time for the lid comparison and the hydrate.

Related simulations and guides

The Mole, Molar Mass and Avogadro's Number – Counting Particles by Weighing Reactants, products and leftovers – sandwiches and molecules

For more chemistry activities, see interactive chemistry lesson ideas. For another mass-based practical, try the making salts virtual lab.