The van't Hoff factor (i) is a measure of the number of particles a substance dissociates into when dissolved in a solvent. For NaCl, i = 2 because it dissociates into Na+ and Cl- ions. For KNO3, i = 2 because it dissociates into K+ and NO3- ions.
The boiling point elevation and freezing point depression of a solution are both colligative properties, which means that they depend on the number of particles in a solution rather than their chemical nature. The boiling point elevation (∆Tb) and freezing point depression (∆Tf) are given by the following equations:
∆Tb = Kb * m * i
∆Tf = Kf * m * i
where Kb is the boiling point elevation constant, Kf is the freezing point depression constant, m is the molality of the solution (moles of solute per kilogram of solvent), and i is the van't Hoff factor.
Since 1 m NaCl has a higher van't Hoff factor than 0.5 m KNO3, it will cause a greater boiling point elevation and freezing point depression. Therefore, the vapor pressure of 1 m NaCl will be lower than that of 0.5 m KNO3.
In summary, the statement that the vapor pressure of 1 m NaCl is lower than that of 0.5 m KNO3 is incorrect. The vapor pressure of 1 m NaCl will actually be higher due to its higher van't Hoff factor.
