The formula to calculate the heat change of a reaction is
ΔH = -mcΔT (where m is the mass of the fluid whose temperature changes which in this case is water, c is the specific heat capacity of water and ΔT is the change in temperature of water)
Heat change when 0.025 mol of Na is added
Now this is heat change for 0.025 mole of Na.
Enthalpy change means you have to calculate the heat change for 1 mole.
So enthalpy change for 1 mole of Na is