Heat per mole
The 4200 J belongs to the amounts that reacted. Dividing by the number of moles gives the heat for one mole.
The heat of reaction
Moles of NaOH in 50 cm3 of 2 mol dm-3 = 2/1000 × 50 = 0.1 mol
Moles of HCl in 50 cm3 of 2 mol dm-3 = 2/1000 × 50 = 0.1 mol
0.1 mol NaOH with 0.1 mol HCl releases 4.2 kJ
1 mol NaOH with 1 mol HCl releases 4.2 kJ ÷ 0.1 mol
= 42.0 kJ mol-1
This is the heat of reaction of NaOH with HCl — an experimental value.
State the physical states
The experiment used aqueous sodium hydroxide. Done with solid sodium hydroxide, NaOH(s), it gives a quite different value. So the heat change of a reaction depends on the physical states (solid, liquid, gas, aqueous) of the reactants and products, and they must be given whenever a heat change is stated.