The concept of deviation from Raoult's law was introduced in a passage in CC (p156). If 2 volatile liquids are mixed and has releases energy (exothermic), there will be a negative deviation of vapour pressure from the predicted value (XaPa + XbPb). Hence, the resultant pressure MAY be lower than either of the pure liquids

521) Please read the passage on CC P156. Raoult's law only predicts the vapour pressure under ideal conditions. It will over or under-estimate the vapour pressure in real life as when you mix two things it will release or absorb energy. If the process is exothermic, the solute-solvent bond is stronger than the solute-solute and solvent-solvent bonds. That means the vapour is less likely to leave the solution, making the real vapour pressure less than that of predicted (which is somewhere in between the vapour pressure of the two).

503) There are 2 steps in forming a solution. First step is to break the bonds that hold the solute-solute and solvent-solvent, the second one is to form the new solute-solvent bonds. In the case of NaCl, the first step is the separation into Na and Cl ions, which is endothermic. The second one is the hydration process, which is endothermic. As given in the stem, the first step would be +786, second step is -783.