[Burner]

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 cm³. The molarity is 0.2M(or mol dm^-3) and the volume is 100cm³, i.e. 0.1 dm³. 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 cm³ to dm³ before the calculation(1 dm³ = 1000 cm³)

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

[jamesbrown]

So is the moles of copper sulphate 0.2*0.1=0.02 moles, or is this wrong and can you also help me with this next question:
the next question was show that magnesium was in excess? can someone explain how to do this? is it empathy change.

[Burner]

So is the moles of copper sulphate 0.2*0.1=0.02 moles

Yes

the next question was show that magnesium was in excess? can someone explain how to do this? is it empathy change.

No, not 'empathy' or enthalpy or such kind of things.

So you have number of moles of Mg=0.083 mol and number of moles of CuSO4=0.02 mol. The reactant in excess is the one which will not be used up when the other reactant(s) is used up. To determine this, you need to write a balanced equation of the reaction between CuSO4 and Mg first.

[jamesbrown]

The balanced equation is: 2CuSO₄ + 2Mg  2MgSo₄ + Cu₂
So whats the next step?

[Burner]

The balanced equation is: 2CuSO₄ + 2Mg  2MgSo₄ + Cu₂

Incorrect. Why do you write Cu₂? Cu exists as metals, not molecules.

[jamesbrown]

2CuSO4 + 2Mg  2MgSo4 + 2Cu
is this right, then tell me what the next step is?

[Borek]

is this right, then tell me what the next step is?

You were told several times this is a limiting reagent problem at this stage, and you were told where to look for hints and explanations. Instead of trying to find an information about how to solve this kind of a problem, you only ask "what is the next step, what is the next step, what is the next step". Sorry, this is going nowhere, you are not going to learn anything this way. I am locking this thread, let it be a lesson for you - nobody is going to spoon feed you for ever.