How to Elevate and Calculate Boiling Points of Solvents
Boiling point elevation refers to the tendency of a solvent’s boiling point to increase when an impurity (a solute) is added to it. In fact, the more solute that is added, the greater the change in the boiling point.
Boiling point elevations are directly proportional to the molality of a solution, but chemists have found that some solvents are more susceptible to this change than others. The formula for the change in the boiling point of a solution, therefore, contains a proportionality constant, abbreviated Kb, which is a property determined experimentally and must be read from a table such as this.
|Solvent||Kb in Degrees C/m||Boiling Point in Degrees C|
The formula for the boiling point elevation is
where m is molality. Note the use of the Greek letter delta in the formula to indicate that you’re calculating a change in the boiling point, not the boiling point itself. You need to add this number to the boiling point of the pure solvent to get the boiling point of the solution. The units of Kb are typically given in degrees Celsius per molality.
Boiling point elevations are a result of the attraction between solvent and solute particles in a solution. Colligative properties such as boiling point elevation depend on only the number of particles in solution. Adding solute particles increases these intermolecular attractions because more particles are around to attract one another. To boil, solvent particles must therefore achieve a greater kinetic energy to overcome this extra attractive force, which translates into a higher boiling point.
Try this example: What is the boiling point of a solution containing 45.2 g of menthol (C10H20O) dissolved in 350 g of acetic acid?
The problem asks for the boiling point of the solution, so you know that first you have to calculate the boiling point elevation. This means you need to know the molality of the solution and the Kb value of the solvent (acetic acid). The table tells you that the Kb of acetic acid is 3.07 degrees C/m. To calculate the molality, you must convert 45.2 g of menthol to moles:
You can now calculate the molality of the solution, taking care to convert grams of acetic acid to kilograms:
Now that you have molality, you can plug it and your Kb value into the formula to find the change in boiling point:
You’re not quite done, because the problem asks for the boiling point of the solution, not the change in the boiling point. Luckily, the last step is just simple arithmetic. You must add your
to the boiling point of pure acetic acid, which, according to the table, is 118.1 degrees Celsius. This gives you a final boiling point of
for the solution.