Here we're going to discuss the phenomenon when adding a solute to a pure solvent which results in the increase of the boiling point of the solvent. Remember, the more solute you add, the higher your boiling point will be. Now, with this idea of boiling point, we have what's called the normal boiling point and then the boiling point of the solution that's created. We're going to talk about the normal boiling point, denoted as bp, which is the boiling point of the pure solvent before the addition of solute. So before any solute has been added, this is basically the boiling point of the pure solvent. Now, the boiling point of the solution, denoted as bpsolution, is the boiling point of the solvent after the addition of the solute. Remember, a solution is formed when we add solute to a solvent.
Next, if we look at boiling point elevation, remember it increases as we add more solute. We have four areas that we need to focus on. The first area deals with the boiling point elevation formula. Here we're going to say Δtb=I⋅kb⋅m, where the change in our boiling point Δtb equals I times kb times lowercase m. I is related to our Van't Hoff factor, and these are our variables. We're going to say here that kb is the boiling point constant of our pure solvent in units of degrees Celsius over molality, and then lowercase m is the molality of the solution in moles of solute per kilogram of solvent. Once we've used this boiling point elevation formula and understand its different variables, we can then determine the boiling point of our solution. Here, we're going to say the boiling point of the solution equals the normal boiling point of our pure solvent, plus Δtb.
The solvents that are customarily used in questions like this are water, benzene, chloroform, or ethanol. Here are their normal boiling points. If we add solute to them, we'd expect their boiling points to be higher than these that are reported here. Their kb values, their boiling point constants, are these values respectively: 0.51, 2.53, 3.60, and 1.20. No, you don't need to memorize these numbers. These are just different types of solvents that you might see pop up in a question dealing with colligative properties.
One last thing, recall if a compound is covalent, nonvolatile, or nonionic, meaning it is not an electrolyte, then its Van't Hoff factor is going to be equal to 1. So just keep in mind when it comes to boiling point elevation, these are the key areas you need to pay attention to in order to find the boiling point of a solution.
