I found out that the moles of copper sulphate was 20, because the volume is 100 and the density is 0.2 so there is 20 moles.

No. You have a 0.2M copper sulphate solution of 100 cm

^{3}. The molarity is 0.2M(or mol dm

^{-3}) and the volume is 100cm

^{3}, i.e. 0.1 dm

^{3}. To calculate the number of moles in a

*solution*, you need to multiply the molarity(concentration) by the volume, but not density. And bear in mind the units, you need to convert cm

^{3} to dm

^{3} before the calculation(1 dm

^{3} = 1000 cm

^{3})

The the next question was what is the moles of magnesium, which I worked out to be 0.083, because ar = 24 and mass = 2.

Good