Consequently, solvents with higher chemical potentials will also have higher vapor pressures. The boiling point is reached when the chemical potential of the pure solvent, a liquid, reaches that of the chemical potential of pure vapor. Because of the decrease in the chemical potential of mixed solvents and solutes, we observe this intersection at higher temperatures.
In other words, the boiling point of the impure solvent will be at a higher temperature than that of the pure liquid solvent. Thus, boiling point elevation occurs with a temperature increase that is quantified using. Freezing point is reached when the chemical potential of the pure liquid solvent reaches that of the pure solid solvent.
Again, since we are dealing with mixtures with decreased chemical potential, we expect the freezing point to change. Unlike the boiling point, the chemical potential of the impure solvent requires a colder temperature for it to reach the chemical potential of the pure solid solvent. Therefore, a freezing point depression is observed.
What is the molar mass of the compound? First we must compute the molality of the benzene solution, which will allow us to find the number of moles of solute dissolved. The freezing point depression is especially vital to aquatic life. Since saltwater will freeze at colder temperatures, organisms can survive in these bodies of water. Road salting takes advantage of this effect to lower the freezing point of the ice it is placed on.
Lowering the freezing point allows the street ice to melt at lower temperatures. Benzophenone has a freezing point of Find the freezing point depression constant for the solvent. The vapor pressure of a solution blue is lower than the vapor pressure of a pure solvent pink. As a result, the freezing point of a solvent decreases when any solute is dissolved into it.
When a pure solvent freezes, its particles become more ordered as the intermolecular forces that operate between the molecules become permanent.
In the case of water, the hydrogen bonds make the hexagonally-shaped network of molecules that characterizes the structure of ice.
By dissolving a solute into the liquid solvent, this ordering process is disrupted. As a result, more energy must be removed from the solution in order to freeze it, and the freezing point of the solution is lower than that of the pure solvent.
The magnitude of the freezing point depression is directly proportional to the molality of the solution. The equation is:. The proportionality constant, , is called the molal freezing-point depression constant. It is a constant that is equal to the change in the freezing point for a 1-molal solution of a nonvolatile molecular solute.
For water, the value of is So the freezing temperature of a 1-molal aqueous solution of any nonvolatile molecular solute is Every solvent has a unique molal freezing-point depression constant. These are shown in Table below , along with a related value for the boiling point called. Ethylene glycol C 2 H 6 O 2 is a molecular compound that is used in many commercial anti-freezes. A water solution of ethylene glycol is used in vehicle radiators to lower its freezing point and thus prevent the water in the radiator from freezing.
Notice that the change in freezing or boiling temperature depends solely on the nature of the solvent, not on the identity of the solute. One valuable use of these relationships is to determine the molecular mass of various dissolved substances. As an example, perform such a calculation to find the molecular mass of the organic compound santonic acid, which dissolves in benzene or chloroform. A solution of 50 grams of santonic acid in grams of benzene boils at Referring to Table.
Rearranging the boiling point equation to yield molality and substituting the molal boiling point constant from Table 1, you can derive the molality of the solution:. That concentration is the number of moles per kilogram of benzene, but the solution used only grams of the solvent. The moles of santonic acid is found as follows:.
The boiling point of a solution was used to determine that santonic acid has a molecular mass of approximately You can also find this value by using the freezing point of the solution.
In the two previous examples, the sucrose and santonic acid existed in solution as molecules, instead of dissociating to ions.
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