To know the density you also need to know the volume of the rock.
When it comes to physical changes like phase changes, there are two types of heat energy: sensible heat and latent heat. Sensible heat is the heat absorbed/released when you heat the substance but it doesn't change phase. An example would be heating lukewarm water. The substance is liquid all throughout. Latent heat, on the other hand, is the heat absorbed/released when there is a phase change. An example would be boiling water, because it changes liquid to vapor.
Hence, for freezing liquid, you use the latent heat, specifically the heat of fusion. The answer should be
2.5 g * (1 mol/18.02 g) * 6.03 kJ/mol = 0.84 kJ/mol
The answer is not in the choices. You only use Hvap if you boil water.
I believe it would be better to use an orbital designation than the written configuration, if the number of electrons in the ground state of the atom are quite high for the given element, as above 50, for instance.
This saves space and also one can see the discrete quantized energy levels associated with the subshells of the main energy levels if written in orbital designation.
