Question

How much heat is required to solidify 2.0 kg of liquid copper?

Answer 1

The latent heat of fusion of copper is $L_f = 209 kJ/kg$, so the amount of heat needed to solidify the copper is this value times the mass of the liquid copper:
$Q=m L_f = (2.0 kg)(209 kJ/kg)=418 kJ$