GCSE · Chemistry · Edexcel · Spec 1CH0

Experiment to determine the empirical formula of magnesium oxide

Burn magnesium and it gets heavier, not lighter. That one surprise hands you the formula MgO.

From bench to formula

Step through the experiment. Every measurement exists so that the next step is possible.

Heat strongly: lid up, lid downHeat the magnesium strongly, lifting the lid carefully and often. Lifting it lets oxygen in so the magnesium can react. Putting it back in between keeps the product from escaping.

Predict, then check

Commit to an answer before you reveal anything.

Mass 3 (24.38 g) is bigger than mass 2 (24.21 g), even though the magnesium was heated strongly. What explains the gain?

Open air or lidded crucible?

Heated in the openvsLidded crucible

Two ways to heat magnesium, each with a trade-off.

Focus

What you get out of it

Heated in the open

Useful qualitative observations

Lidded crucible

Quantitative measurements you can calculate from

The insight

Open heating shows you what happens. The lidded crucible gives you numbers, and numbers are what a formula calculation needs.

Watching it happen

Heated in the open

Very fast and easy to observe

Lidded crucible

Not as easily observed

Bright light

Heated in the open

Produces a bright light that can damage eyes

Lidded crucible

Protects the chemist from the bright light

Product

Heated in the open

Some of the product can easily be lost

Lidded crucible

Loses less product

Set-up

Heated in the open

Open: substances can freely enter and leave

Lidded crucible

Closed, but occasionally opened

Lid and limiting reactant

What should you do with the lid?

You are heating magnesium in a crucible with a lid. You want all of it to turn into magnesium oxide, and you want to weigh all of that oxide at the end.

Which idea is closest to what you would do?

Take the lid off completely. More oxygen means a better result.Lift the lid carefully and often, but never leave it off for long. Magnesium should run out first, with oxygen in excess.Keep the lid shut the whole time, so nothing can escape.Better if the oxygen runs out first, so none is wasted.

How sure are you?

I'm sureI think soI'm guessing

Take the lid off completely. More oxygen means a better result.

This is a common misconception

Tempting, but an uncovered crucible lets product be lost, and then mass 3 is too small. The lid is lifted to let oxygen in, not left off.

Lift the lid carefully and often, but never leave it off for long. Magnesium should run out first, with oxygen in excess.

This matches the evidence

Yes. Lifting often lets enough oxygen in to oxidise all the magnesium, and not leaving it off keeps the product in. Magnesium is the limiting reactant and oxygen is in excess, so every magnesium atom reacts.

Keep the lid shut the whole time, so nothing can escape.

This is a common misconception

That protects the product, but not enough oxygen gets in to completely oxidise the magnesium. The lid has to be lifted often enough for enough oxygen to enter.

Better if the oxygen runs out first, so none is wasted.

This is a common misconception

If oxygen ran out first, some magnesium would be left unreacted. The magnesium has to be the limiting reactant, the one that completely reacts, with oxygen in excess.

Lift the lid often, never leave it off long, and keep oxygen in excess so the magnesium is the reactant that runs out.

Your turn

The same method on iron oxide

A compound contains 0.70 g of iron and 0.30 g of oxygen. Use Fe = 56 and O = 16 to find its empirical formula.

Write down the masses: iron 0.70 g, oxygen 0.30 g.
missing step

Which line is step 2?

Divide each mass by its relative mass: iron 0.70 ÷ 56 = 0.0125 mol, oxygen 0.30 ÷ 16 = 0.01875 mol.Multiply each mass by its relative mass: iron 0.70 × 56 = 39.2, oxygen 0.30 × 16 = 4.8.Divide each relative mass by the mass: iron 56 ÷ 0.70 = 80, oxygen 16 ÷ 0.30 ≈ 53.3.

Chemistry · Reaction balancer

From mole ratio to balanced equation

Set the coefficients until the atoms match on both sides.

Reactants

O₂

Products

Na₂O

Na1→2✗

O2→1✗

Conservation check: count atoms on both sides.Not yet.

A 4 : 1 ratio of sodium to oxygen gives the first two coefficients. Each Na₂O has two sodium atoms and one oxygen atom, so you need 2Na₂O to balance both: 4Na + O₂ → 2Na₂O.

Tap + or − under each molecule to set its coefficient.

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

Burn magnesium, weigh the crucible, and the formula falls out of the numbers.

What you need to know

Heating magnesium in air is an oxidation reaction: oxygen is a reactant, so magnesium + oxygen → magnesium oxide.
So the contents of the crucible gain mass. By conservation of mass, that gain is the oxygen that reacted.
Have a goA crucible, lid and magnesium weigh 31.20 g. After heating and cooling they weigh 31.45 g. How much oxygen reacted?
0.25 g
The contents gained 0.25 g, and that gain is the oxygen that bonded to the magnesium (assuming no product escaped).
A crucible with a lid loses less product, protects you from the bright light and gives measurements you can calculate from.
Lift the lid often enough for oxygen to get in, but never leave it off so long that product escapes.
Have a goDev lifts the lid, counts to sixty, then wonders why his magnesium oxide weighs so little. What would you tell him?
The lid was off too long, so some product escaped.
Lifting the lid lets oxygen in, but leaving it off for too long lets product be lost, which shrinks mass 3.
Magnesium must be the limiting reactant with oxygen in excess, so that every magnesium atom reacts.
Weigh three times: crucible and lid, then with magnesium, then with the oxide. Heat until the mass stops changing.
Have a goZed heats his magnesium once, weighs it, shouts “Done!” and leaves. What should he have done?
Heated again and reweighed until the mass stopped changing.
A constant mass is the sign that all the magnesium has reacted, so you check and reheat if you need to.
Subtract to find each mass: magnesium is mass 2 − mass 1, magnesium oxide is mass 3 − mass 1, oxygen is the difference.
Moles = mass ÷ relative mass. Divide by the smallest, then round or multiply to get a whole-number ratio.
An empirical formula is the simplest whole-number ratio of atoms. A molecular formula gives actual numbers: C₂H₆ becomes CH₃.
Know the mole ratio and you know the balanced equation: the coefficients are the numbers that balance each element’s atoms.

The big picture

Heating magnesium in a crucible is an oxidation reaction: magnesium + oxygen → magnesium oxide. Weigh the crucible and lid, then with the magnesium, then after heating, and three masses give you everything. Subtract to find the magnesium and the magnesium oxide, use conservation of mass to find the oxygen, divide each mass by its relative mass to get moles, and simplify the ratio. Here it comes out at about 1 : 1, so the empirical formula is MgO. The same method works for any empirical formula question.

Key points

1Heating magnesium in air is oxidation: magnesium + oxygen → magnesium oxide.

2Three masses: mass 1 crucible + lid, mass 2 + magnesium, mass 3 + magnesium oxide.

3Oxygen mass = magnesium oxide mass − magnesium mass, by conservation of mass.

4Moles = mass ÷ relative mass; divide by the smallest to get a whole-number ratio.

5The experiment gives about 1 : 1 magnesium to oxygen, so the empirical formula is MgO.

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Worked example

Problem

A compound is 71.4% calcium and 28.6% oxygen by mass. Use Ca = 40 and O = 16 to find its empirical formula.

⚠ Watch out

Thinking burning always loses mass. Here the magnesium gains oxygen, so the crucible gets heavier, and that gain is what lets you calculate the oxygen. Also: rounding 1.5 instead of multiplying everything by 2.

🧠

Memory hook

Weigh, subtract, divide, simplify: grams → moles → ratio → formula.

✓

Check yourself

A run gives 0.36 g of magnesium and 0.24 g of oxygen. Without looking back: what do you do to each number first, and what does the ratio tell you?

Flashcards

(12)

What is an oxidation reaction?

A reaction in which oxygen is a reactant: oxygen atoms become bonded to the elements of the reactants.

Word equation for heating magnesium in air?

Magnesium + oxygen → magnesium oxide (2Mg + O₂ → 2MgO).

In the crucible experiment, what are mass 1, mass 2 and mass 3, and what do you subtract?

Mass 1: crucible and lid. Mass 2: plus magnesium. Mass 3: plus magnesium oxide after heating. Magnesium = mass 2 − mass 1; magnesium oxide = mass 3 − mass 1.

How do you find the mass of oxygen that reacted, and why does it work?

Oxygen = mass of magnesium oxide − mass of magnesium, because total mass of reactants equals total mass of products.

Why heat the magnesium until the mass is constant?

It shows all the magnesium has reacted. Check the mass and heat again if necessary.

Why use a crucible with a lid instead of heating in the open?

It loses less product, protects the chemist from the bright light, and gives quantitative measurements. The cost: it is not as easily observed.

What is the lid rule?

Lift it often enough for enough oxygen to get in, but not left off so long that product is lost.

Which reactant must be limiting in this experiment, and which in excess?

Magnesium is limiting (it completely reacts) and oxygen is in excess, so every magnesium atom forms magnesium oxide.

Moles from mass?

Moles = mass in grams ÷ relative mass.

Steps from masses to an empirical formula?

Divide each mass by its relative mass, divide all answers by the smallest, then round or multiply (e.g. by 2 for about 0.5) to a whole-number ratio.

Empirical formula vs molecular formula?

Empirical: simplest whole-number ratio of atoms. Molecular: the actual number of atoms. Ethane is C₂H₆ with empirical formula CH₃; water is H₂O for both.

How does a 4 : 1 mole ratio of sodium to oxygen become an equation?

The coefficients are the numbers that balance the atoms: 4Na + O₂ → 2Na₂O.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

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What should you do with the lid?

The same method on iron oxide

From mole ratio to balanced equation

More Edexcel GCSE Chemistry topics