Certain 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 dissolved in the solvent, not on the identity of the particles. A particle, in this instance, is defined as an ion or a molecule. This experiment focuses on the property of freezing-point depression.

When a particular solute is dissolved in a solvent, the following expression holds true:

ΔT = T_f° - T_f = Kfm

The terms T_f° and T_f refer to the freezing-point temperatures of the pure solvent and the solution, respectively. The term "m" indicates the molality of the solution, which is defined as the number of moles of solute per 1,000 g of solvent. This quantity is used, rather than molarity, because it is not temperature dependent. The constant, K_f, is referred to as the freezing-point-depression constantand is dependent only upon the solvent. The change in temperature is also dependent upon the number of solute particles in solution - the more particles present, the larger the change in temperature.

For this reason, the previous equation is sometimes written as:

T_f° - T_f = K_fim

where i = the number of solute particles produced per formula unit that dissolves. In a solution containing an electrolyte, each ion is considered to be a particle.

This experiment uses cyclohexane, an organic compound that is a liquid at room temperature, as the solvent. The unknown compound is a non-ionic organic molecule; therefore, i is equal to 1. The molecular weight of this unknown compound can be determined by observing the freezing point of a solution of the compound in cyclohexane and comparing it to the freezing point of pure cyclohexane.

The compound cyclohexane has a melting point (or freezing point) of about 6 °C. A series of temperatures of pure cyclohexane are obtained as it cools down from room temperature through its freezing point in an ice bath. These temperatures are then plotted as a function of time. Similarly, temperatures of a solution of the unknown compound dissolved in cyclohexane are obtained as it cools down to the freezing point, which are also plotted. The plots should look similar to the plots in Figure 1. The T_f° and T_f values can be extrapolated, as shown. In Figure 1b, the temperature does not remain entirely constant as the solution freezes. The freezing point of the solution is the point at which it first begins to freeze and is indicated graphically by a change in the slope of the temperature-time curve.

The molality, m, of a solution can be expressed in terms of the molar mass of the solute:

Substituting this expression into the equation for freezing-point depression (where i = 1), obtains:

Rearranging to solve for molar mass, obtains:

The molecular weight (in amu) of a substance has the same numerical value as its molar mass.

The unknown substance is one of the following compounds:

-Biphenyl (C₁₂H₁₀)

-2-Bromochlorobenzene (C₆H₄BrCl)

-Naphthalene (C₁₀H₈)

-Anthracene (C₁₄H₁₀)

-1,4-Dibromobenzene (C₆H₄Br₂)

Figure 1. Figure 1a is a plot of temperature as a function of time for the determination of T_f° for the pure solvent. Figure 1b is a plot of temperature as a function of time for the determination of T_f for the solution.