Chemistry - % Mass and Empirical Formula Calculation

Here is some help on BBC Bitesize for these two types of calculation which we looked at yesterday:
http://www.bbc.co.uk/schools/gcsebitesize/science/add_aqa_pre_2011/chemcalc/chemcalc_bothrev5.shtml
http://www.bbc.co.uk/schools/gcsebitesize/science/add_aqa_pre_2011/chemcalc/chemcalc_higherrev2.shtml
Youtube provides a friendly science teacher to explain empirical formulae if you need further refreshment.

Your homework for Friday afternoon is to find one hard % composition question, and one hard empirical formula question from the sheets I've already given you, and have a go at these.

If you can't track down a relevant sheet, have a go at these:

1) What is the percentage of Oxygen in Na2So4?
2) What is the empirical formula of a substance that contains 18g of carbon, 4.5g of hydrogen and 12g of oxygen?