Chesapeake Campus – Chemistry 112 Laboratory
Objectives
- Define and identify colligative properties.
- Observe the effect of a solute on the freezing point of a solvent.
- Determine the molar mass of a nonvolatile, nonionizing solute.IntroductionColligative Properties
The properties of a solution differ from those of a pure solvent due to interactions that take place between the solute and solvent molecules. The properties that exhibit such changes are called the colligative properties and include vapor pressure lowering, boiling point elevation, freezing point depression and changes in osmotic pressure. These properties are dependent only upon the number of particles (ions or molecules) that are dissolved in the solvent and not on the identity of the particles.Colligative properties are utilized in many real-world scenarios. When the forecast calls for snow, the roads are often salted. The salt will dissolve into the snow and allow the freezing point of water to lower so that solid snow turns to water and road conditions are safer. The fact that salt ionizes increases the impact the salt will have. Similarly salt is often added to a pot of water. By adding a solute the water will boil at a higher temperature and will cook the pasta faster.Freezing Point Depression
This experiment will examine the phenomenon of freezing point depression. When a particularsolute is dissolved in a solvent, the following expression holds true:
T=Tf –Ti =iKfm
The terms Tf and Ti refer to the freezing-point temperatures of the pure solvent and the solution, respectively. The term i is the van’t Hoff Factor and refers to the number of solute particles produced per formula unit that dissolves. Therefore, the more particles that are present, the larger the change in freezing point that results. In a solution containing an electrolyte, each ion is considered to be a particle. The constant, Kf, is referred to as the freezing-point-depression constant and is dependent only upon the solvent. Table 1 gives the Kf values for several solvents. The term “m” indicates the molality of the solution, which is defined as the number of moles of solute per kg of solvent. This quantity is used, rather than molarity, because it is not temperature dependent.
Table 1
Kf (◦C/m) |
Freezing Point (◦C) |
Kb (◦C/m) |
Boiling Point (◦C) |
|
Chloroform |
4.68 |
-64 |
3.62 |
61 |
Cyclohexane |
20.0 |
6.0 |
2.75 |
81 |
Ethanol |
— |
-114 |
1.16 |
78 |
Water |
1.86 |
0 |
0.515 |
100 |
You will explore the influence of i on the freezing point depression in Part C of the experiment. In Part A of the experiment, you will determine the freezing point of your pure solvent. While the freezing point of water is well recorded, it is important to include this step because it helps calibrate your temperature probe for the rest of the experiment. In Parts B and C, you will be able to determine the molecular weight of an unknown compound by observing the freezing point of a solution of the compound in water and comparing it to the freezing point of pure water. Commonly used solutes are included in Table 2.
Compound |
van’t Hoff Factor |
Molar Mass (g/mol) |
Ethanol |
1 (covalent) |
46.07 |
Glucose |
1 (covalent) |
180.16 |
Maleic Acid |
1 (covalent) |
116.072 |
Sucrose |
1 (covalent) |
342.30 |
Calcium Chloride (anhydrous) |
3 (ionic) |
110.98 |
Potassium Chloride |
2 (ionic) |
74.55 |
Sodium Chloride |
2 (ionic) |
58.44 |
Sodium Phosphate |
4 (ionic) |
163.94 |
Table 2: Possible Solutes for Part B (Covalent) and Part C (Ionic)
Cooling Curves
Collecting the temperature curve for a pure solvent is usually straightforward.
Figure 1a indicates what you can expect the cooling curve for water to look like.
However, when collecting your cooling curves of solutions, you can often expect to see super cooling or a temperature decrease below the freezing point. This decrease only lasts a short time and will then increase back to the freezing point. You want to record the freezing point as the temperature in the graph that is maintained while both liquid and solid phases are present. Figure 1b shows what you can expect from your cooling curve for a solution.
Logger Pro
Each Logger Pro has its own temperature probe. Use the same probe for the entire experiment to minimize errors. Before you turn on the probe, plug in the temperature probe. Once the Logger Pro is turned on, you can select the unit the temperature is recorded in. It should already be set to C. Record all values given to give the most significant figures.