The boiling point elevation of a solution is a colligative property, which means that it depends on the number of particles dissolved in the solvent, and not on the identity of the particles.

The boiling point elevation is directly proportional to the molality of the solution, which is defined as the number of moles of solute per kilogram of solvent. In this case, both NaCl and MgCl2 are dissolved in water at a concentration of 1 mol, so they have the same molality.

However, MgCl2 dissociates into three ions in water (Mg2+ and 2 Cl-), while NaCl dissociates into only two ions (Na+ and Cl-). This means that MgCl2 produces more particles in solution than NaCl, and therefore has a greater boiling point elevation.