Calculating Empirical Formulas

Question 1

How can you work out the empirical formula from masses?

Answer 1

1) Work out the number of moles you have, by dividing the masses by Ar.
2) Divide all the numbers by the biggest whole number that goes into both.

Question 2

What experiment can you do to calculate the empirical formula of a metal oxide (e.g. magnesium oxide)?

Answer 2

1) Get a crucible and heat it until it’s red hot. (This will make sure it’s clean and there are no traces of oil or water from previous experiments.)
2) Leave the crucible to cool, then weigh it, along with its lid.
3) Add some clean magnesium ribbon to the crucible. Reweigh the crucible, lid and magnesium ribbon. The mass of magnesium is this reading minus the initial reading.
4) Heat the crucible containing the magnesium. Put the lid on the crucible to stop any bits of solid from escaping, but leave a small gap to allow oxygen to enter the crucible..
5) Heat the crucible strongly for around 10 minutes, or until all the magnesium ribbon has turned white.
6) Allow the crucible to cool and reweigh the crucible with the lid and its contents. The mass of magnesium oxide you have is this reading, minus the initial reading for the mass of the crucible and lid.