Experiment: Empirical formula of magnesium oxide

Finding the empirical formula of magnesium oxide is the classic experiment for turning the mole concept into a laboratory result. By combining a metal with oxygen and weighing before and after, you measure the masses that react and convert them to a mole ratio, which is exactly what an empirical formula is. It is one of the most examined practicals in The Mole Concept, Chemical Formula and Equation chapter. This guide sets out the method, the observations, and the Paper 3 skills the practical assessment rewards.

Aim

To determine the empirical formula of magnesium oxide by finding the ratio of the number of moles of magnesium atoms to the number of moles of oxygen atoms that combine when magnesium is heated in air.

Apparatus and materials

• Magnesium ribbon
• Crucible with a lid
• Pipe-clay triangle
• Tripod stand and Bunsen burner
• Tongs
• Electronic balance
• Sandpaper (or emery paper)
• Heat-resistant mat

Procedure

1. Clean a length of magnesium ribbon with sandpaper to remove the oxide layer, then coil it loosely.
2. Weigh the empty crucible together with its lid and record the mass.
3. Place the coiled magnesium in the crucible, weigh the crucible, lid and magnesium, and record the mass.
4. Set the crucible on the pipe-clay triangle on the tripod and heat it strongly.
5. When the magnesium begins to burn, use tongs to lift the lid slightly at intervals to let more air in, then replace it quickly to stop the white smoke escaping.
6. Continue heating until all the magnesium has reacted and only a white ash remains.
7. Allow the crucible to cool, then weigh the crucible, lid and contents and record the mass.
8. Heat the crucible again for a few minutes, cool it, and reweigh it.
9. Repeat the heating, cooling and weighing until the mass no longer changes (constant mass), and record the final mass.

Expected observations

The magnesium ribbon burns with a bright, dazzling white flame and gives off white smoke. The shiny grey ribbon is replaced by a soft white powder or ash, which is magnesium oxide. The mass of the contents after burning is greater than the mass of the magnesium at the start, because oxygen from the air has combined with the magnesium.

Inference and conclusion

The increase in mass is the mass of oxygen that has combined with the magnesium. The mass of magnesium is found from the difference before heating, and the mass of oxygen from the increase in mass after heating. Dividing each mass by its relative atomic mass gives the number of moles of magnesium atoms and of oxygen atoms; expressing these as the simplest whole-number ratio gives the empirical formula. The ratio works out as one mole of magnesium to one mole of oxygen, so the empirical formula of magnesium oxide is MgO, formed according to the equation 2Mg + O₂ → 2MgO.

Science process skills (Paper 3 style)

Making a hypothesis. When magnesium is heated in air it combines with oxygen in a fixed ratio, so a fixed mass of magnesium always combines with the same mass of oxygen.

Identifying variables. The manipulated variable is the mass of magnesium used; the responding variable is the mass of magnesium oxide formed; the controlled variables include the supply of air and heating to constant mass.

Tabulating data. Draw a table for the mass of the crucible and lid, the mass of the crucible, lid and magnesium, and the mass of the crucible, lid and magnesium oxide, so the mass of magnesium and the mass of oxygen can be calculated by subtraction.

Making inferences. Explain why lifting the lid at intervals is necessary: it lets in oxygen so the reaction is complete, while replacing the lid stops the white magnesium oxide smoke escaping and lowering the measured mass.

Operational definition. The reaction is operationally defined as complete when repeated heating no longer changes the mass of the crucible and its contents.

Safety precautions

• Do not look directly at the burning magnesium, because the intense white light can damage the eyes.
• Wear safety goggles and handle the hot crucible only with tongs, to avoid burns.
• Place the hot crucible on a heat-resistant mat and never on the bench directly, to prevent damage and burns.
• Keep the lid nearby throughout, so the burning magnesium can be covered if the reaction becomes too vigorous.

Common errors

• Opening the lid too wide or too often. This lets the white magnesium oxide smoke escape, so the mass of oxygen recorded is too small and the ratio is wrong.
• Not cleaning the ribbon. An oxide coating adds mass that is not pure magnesium, changing the calculated ratio.
• Stopping before constant mass. If heating stops while some magnesium is unreacted, the mass of oxygen is too small.
• Ignoring magnesium nitride. Some magnesium reacts with nitrogen; keeping air flowing and heating fully reduces this side reaction.

How our teachers use this experiment

In one-to-one SPM Chemistry lessons, taught in English from RM50 per hour, we use this experiment to join the practical to the calculation: a student who understands why the lid is lifted and why heating continues to constant mass can also set out the mole-ratio working that earns full marks. Because empirical-formula questions recur across SPM Chemistry and the method is a standard Paper 3 practical (Paper 3 is a practical test assessing science process skills), securing both the technique and the calculation here protects marks in Paper 2 as well.