if the solute is MgSO4 and it is disolved in 800g water water are the ideal melting and boiling pionts of the solution?
I assume you intended to ask "WHAT are the ideal.....". You have no amount of MgSO4 given; therefore, calculation of the boiling point and freezing point is not possible. If you wish it can be done this way.
mols MgSO4 = grams/molar mass.
Then molality = mols MgSO4/kg solvent (0.800 kg). Then delta T = iKf*m for freezing point difference and subtract from zero. For boiling point, delta T = iKb*m and add to 100 C. i = 2 in both cases.