Experiment 17
Colligative Properties
To prepare for this experiment:
Carefully read the entire experimental guide (below).
Answer all the prelaboratory practice problems.
Review your Chapter 13 lecture notes (particularly the section on boiling point
elevation) and bring them to lab.
Review Experiment 3 (Graph Drawing) and bring your thermometer calibration
curve to lab.
Introduction
Colligative properties are those properties of a solution that depend on the number
of molecules or ions dissolved in a solution, and not on the identity of the species in
solution. Examples of these properties are boiling point elevation, freezing point
depression, and osmotic pressure. In this experiment, you will study boiling point
elevation using sodium chloride, NaCl. You may recall from your notes that boiling
point elevation is described by the equation:
mbb ciKT
where ∆Tb is the boiling point elevation, i is the van’t Hoff factor, Kb is the boiling
point constant of the solvent, and cm is the molality of the solution. The boiling point
elevation, ∆Tb, is the difference between the boiling point of the pure solvent and the
boiling point of the solution. In theory, a solution of NaCl has a van’t Hoff factor of 2, a
solution of MgCl2 has a van’t Hoff factor of 3, and a solution of a non-dissociating
substance like sugar would have a van’t Hoff factor of 1.
In this experiment, you will be creating 3 salt/water solutions of varying
molalities. You will use a hot plate to heat up your solutions to the boiling point and
measure this temperature using a thermometer. Using this data, you will determine the
boiling point constant.
Prelab Questions
1. The boiling point elevation constant of carbon tetrachloride is 4.95 ºC/m. The
density of carbon tetrachloride is 1.584 g/cm3. The normal boiling point of
carbon tetrachloride is 76.8ºC.
a. Calculate the boiling point of a solution made by mixing 25.56 grams of
solid iodine with 250 ml carbon tetrachloride. (Show all work)
b. Calculate the boiling point of a solution made by mixing 25.56 grams of
solid naphthalene (C10H8) with 250 ml carbon tetrachloride. (Show all
work)
