Sorry, but I start getting a little confused.
then 100cm3 of 0.20 mol/dm3 copper sulphate solution was put in a calorimeter and 2.0g of magnesium power added. The temperature of the solution rose by 25.1 °C.
Number of moles of magnesium= 2/24.3 = 0.0823 mol
Number of moles of copper sulphate= 0.1*0.2= 0.02 mol
Magnesium is in excess. Not all magnesium will be dissolved. Also, some copper metal is formed too. Calculating the mass in this case can be complex, is it (100g + mass of magnesium unreacted + mass of copper formed)? Please someone answer this.