Which solution has the highest boiling point?

Question

.40 M NaCl

.40 M C₆H₁₂O₆

.40 M MgCl₂

Answer 1

Boiling point is a colligative property which means it depends on the number of particles dissolved in solution. Even though all 3 solutions have the same concentration, they all have different number of particles. To understand this, we have to consider how the solutes behave when dissolved in water.

Assuming water is your solvent, NaCl (ionic) will split up into 2 ions, Na+ and Cl-. By contrast, C6H12O6 (covalent) will not dissociate into any particles. This means, for the same concentration of NaCl and C6H12O6, the NaCl solution will have twice as many particles in solution than the C6H12O6. Therefore, NaCl will have a higher BP.

Now let's look at MgCl2. MgCl2 is ionic and will dissociate into 3 parts; Mg+2 and 2 Cl-. The number of particles in the MgCl2 solution will be the greatest of the three solutions and will have the highest BP.

To relate this back to equations, you probably have a formula to calculate the change in boiling point that looks like: deltaT = i Kb m

In this formula, 'i' relates to how many particles the solute breaks up into when dissolved in water. For NaCl, i=2. For C6H12O6, i=1. For MgCl2, i=3.

Sidenote: Check to see if your teacher actually gave you 0.40 M or 0.40 m. The difference is subtle but Molarity is not the same as molality. Colligative properties use molality.