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Unformatted text preview: 53 Graph showing the freezing points of the pure substance, the freezing point of the mixture, the change in temperature ( Δ T), and the supercooling effect Molar Mass Determination by Freezing Point Depression If a nonvolatile solute is added to a liquid, a number of physical properties of the pure substance change, including vapor pressure depression, freezing point depression, and boiling point elevation. These alterations are collectively known as colligative properties of solutions. The colligative properties of a solution change in proportion to the concentration of solute dissolved in solvent. These properties depend only on the number of solute particles present in a given amount of solvent and not on the type of particles dissolved. Therefore, the concentration of the solute is most conveniently expressed in terms of molality ( m ) molality (m) = moles of solute kg of solvent The change in the freezing point ( Δ T f ) in ˚C for a nonvolatile organic solvent can be determined using the following equation, where k f is characteristic for the solvent used: Δ T f = k f m We can determine the molar mass of the solute using this equation by measuring the change in the freezing point of the solution and solving the equation for molality. The calculated molality can be used to determine the moles of solute that in turn can be used to calculate the molar mass (grams / mole) of the solute. When a pure substance is heated to a liquid state and allowed to cool, initially the temperature will fall quite rapidly. As the substance approaches its freezing point , solid will begin to form and the temperature will begin to level. The freezing point of the pure liquid is the constant temperature observed while the liquid is solidifying. The cooling behavior of a solution is somewhat different from that of a pure liquid. The temperature at which the solution begins to freeze is lower (i.e. depressed ) than for the pure solvent. Additionally, there is a slow gradual fall in temperature as freezing proceeds. The change in temperature, Δ T , between the freezing point of the pure substance and the freezing point of the mixture is used to calculate the molality of the solution. In both the pure liquid and the solution, a supercooling effect may be seen. As the solid begins to form, the temperature may drop below the actual freezing point initially and then come back up to the freezing point temperature as the solid forms. Supercooling is usually not observed if adequate churning of the sample is provided. When determining the freezing point, the super-cooling effect should be ignored if present; see the figure below. 54 In this experiment, you will first determine the freezing point of a pure solvent, namely cyclohexane, using a salt water ice bath. Next, you will use a known solute, naphthalene (C 10 H 8 ), to determine the k f freezing point constant...
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- Summer '11
- Solvent, Freezing-point depression