Molecule for molecule, the solute that raises the boiling point of water the most is the one that makes the most particles in the solution. Lithium chloride breaks up into two ions (Li+ and Cl-). So does sodium chloride (Na+ and Cl-). Molecular molecules don't break up at all, so sucrose has only 1 particle per molecule. Sodium phosphate makes 4 total particles (3 Na+ ions and 1 PO4^3-). And magnesium bromide would make 3 particles (1 Mg2+ and 2 Br-). So the most is 4.
