Finding the formula of magnesium oxide- As practical chemistry

Finding the formula of magnesium oxide

Aims (Eye protectionmust be worn )

When magnesium is heated in air, it reacts with oxygen. During this oxidation reaction, magnesium oxide is produced. This increases the mass. If we know the mass of magnesium at the start, and the mass of magnesium oxide produced at the end, we can work out the mass of oxygen which has been combined with the magnesium. We can use these masses to work out the formula of magnesium oxide.

Apparatus
Goggles
Bench mat
Tripod
Bunsen burner
Pipe-clay triangle
Crucible and lid
Tongs


10cm length of magnesium ribbon
Small piece of sandpaper
Digital balance


Methods



1. Weigh the empty crucible with its lid, and write down the result in a table.
2. Clean the piece of magnesium ribbon with sandpaper, then coil it loosely around a pencil.
Put the magnesium ribbon into the crucible and put the lid on.
Weigh the crucible, lid, and magnesium together, and write down the result in your table.
3. Put the crucible onto the pipe-clay triangle. Leave the lid slightly ajar to to allow air into the crucible. Heat gently for a minute, then strongly. Continue heating until the reaction has finished (the magnesium will glow at first, then look a bit like a furry grey-black caterpillar – it really will!)


4. Turn the Bunsen burner off, and allow the crucible to cool for a few minutes.
Reweigh the crucible with its lid and contents, and write down the result in your table. cruciblepipe-claytriangletripodHEAT

Results (suggested table only – do not write on this sheet)

crucible + lid =
crucible + lid + magnesium =
crucible + lid + contents after reaction =


Conclusions


1. Work out the mass of magnesium used in the experiment. Apparatus to oxidise magnesium

2. Work out the mass of magnesium oxide formed.

3. Use your answers to (1) and (2) to work out the mass of oxygen gained.

4. Look up Ar(Mg) and Ar(O). Use these numbers, and the masses worked out in parts (1) and (3), to work out the number of moles of magnesium and oxygen involved.

5. Finally, work out your formula for magnesium oxide. You will probably find that the formula has a complicated number in it – round it off to 1 decimal place.

6. The accepted formula is MgO. How close did you get?

Explain why your formula might be different from the accepted one.