Freezing point depression constant nonelectrolyte

ATt is the number of degrees that the freezing point has been lowered (the difference in the freezing point of the pure solvent and the solution). Kt is the freezing-point depression constant (a constant of the individual solvent). The molality (m) is the molality of the solute, and i is the van t Hoff factor, which is the ratio of the number of moles of particles released into solution per mole of solute dissolved. For a nonelectrolyte such as sucrose, the van t Hoff factor would be 1. For an electrolyte such as sodium sulfate, you must take into consideration that if 1 mol of Na2S04 dissolves, 3 mol of particles would result (2 mol Na+, 1 mol SO) ). Therefore, the van t Hoff factor should be 3. However, because sometimes there is a pairing of ions in solution the observed van t Hoff factor is slightly less. The more dilute the solution, the closer the observed van t Hoff factor should be to the expected one. 

Solutions have lower freezing points than the pure solvent. The freezing point elevation (ATf) is directly proportional to the solvent s freezing point depression constant (Kf) times the molality (m) of the nonelectrolyte solute in moles per kg of solvent ... 

The freezing point depressions of solutions of nonelectrolytes have been found to be equal to the molality of the solute times a proportionality constant called the molal freezing point depression constant, Kf. 

Freezing point depression constant, Kf A constant that corresponds to the change in freezing point produced by a one-molal ideal solution of a nonvolatile nonelectrolyte. 

A solutions freezing point depression, ATf, is the difference in temperature between its freezing point and the freezing point of its pure solvent. Molal freezing point depression constants (Kf) for several solvents are shown in Table 14.6. For nonelectrolytes, the value of the freezing point depression is directly proportional to the solutions molality. 

Challenge A 0.045m solution (consisting of a nonvolatile, nonelectrolyte solute) is experimentally found to have a freezing point depression of 0.08°C. What is the freezing point depression constant (/Cf). Which is most likely to be the solvent water, ethanol, or chloroform ... 

If 1.15 g of an unknown solid nonelectrolyte is dissolved in 10.1 g of naphthalene, the resulting solution is found experimentally to freeze 4.3°C lower than pure naphthalene. The freezing point depression constant for naphthalene is 6.85°C/in. Calculate each of the following. 

Kb is the boiling point elevation constant, and for water equals 0.52°C/m. Each solvent has its own unique value for Kb, and the value of Kb for water indicates that a 1.0 m solution of glucose, a nonelectrolyte, would boil 0.52°C higher than that of pure water, 100.52°C. As with the equation used to calculate freezing point depressions, if the solute is an electrolyte, the molality of the ions will be a whole number multiple of the molality of the compound. 

Molar mass determination by freezing-point depression or boiling-point elevation has its limitations. Equations (14.5) and (14.6) apply only to dilute solutions of nonelectrolytes, usually much less than 1 mol kg . This requires the use of special thermometers so that temperatures can be measured very precisely, say to 0.001 °C. Because boiling points depend on barometric pressure, precise measurements require that pressure be held constant. As a consequence, boiling-point elevation is not much used. The precision of the freezing-point depression method can be improved by using a solvent... 

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