Calculate boiling point elevation.
Adding a non-volatile solute lowers the solvent's vapour pressure, so the solution must be heated further before its vapour pressure matches atmospheric pressure. The elevation depends only on how many dissolved particles are present per kilogram of solvent, not on what they are — which is why the van't Hoff factor matters: sodium chloride dissociates into two ions and therefore elevates the boiling point roughly twice as much as an equal molality of sugar.
Boiling point elevation
delta Tb = i x Kb x molality; solution boiling point = normal boiling point + delta Tb
Use 1 for non-electrolytes such as glucose or urea, 2 for NaCl or KCl, 3 for CaCl2 or Na2SO4. Real values fall slightly below the ideal figures at higher concentrations because of ion pairing.
Barely. A tablespoon of salt in five litres of water is about 0.06 molal, which raises the boiling point by roughly 0.06 C. Salt is added for flavour, not for cooking temperature.
Because molality is defined per kilogram of solvent and therefore does not change with temperature, whereas molarity is per litre of solution and shifts as the liquid expands on heating.