Constants for Calculation of Freezing Point Depression

CRYOSCOPIC CONSTANTS FOR CALCULATION OF FREEZING POINT DEPRESSION... 

Cryoscopic Constants for Calculation of Freezing Point Depression... 

The freezing point depression constant for water is known from experiments and can be found in tables Tf = 1.858 ° C kg/mol. To calculate the freezing point, we must first determine the molality of the... 

The depression of the freezing point of a solvent due to the presence of a dissolved solute is an example of a colligative property, that is, a property of a dilute solution that depends on the number of dissolved particles and not on the identity of the particles. Water has a freezing point depression constant, Kf, of 1.86 K kg mol-1. In other words, for every mole of nonvolatile solute dissolved in a kilogram of water, the freezing point of water is lowered by 1.86°C. The change in freezing point, A T, can be calculated from the equation... 

A solution also exhibits a depression in its freezing point. The freezing point depression is the decrease in the temperature of the freezing point due to the addition of a solute. It is calculated using the equations ATj. = Kjm, where ATj. is the decrease in freezing point for the solution, Kj. is the molal freezing point depression constant, and m is the molality of the solution. Water s K. value is 1.86°C/m. 

Using the average freezing point depression constant obtained for lauric acid in your experiment, calculate what would be the freezing point of a 10.0% w/w benzoic acid solution ... 

By taking the slope of the plot of AG/T vs. 1/T, one can determine the enthalpy change for a reaction. The equilibrium constants at different temperatures under constant pressure, freezing point depression, and boiling point elevation may be calculated from Equation (1.116), as will be discussed in Chapter 3. 

Ionization constants for weak acids (and bases) must be calculated from experimentally determined data. Measurements of pH, conductivity, or depression of freezing point provide data from which these constants can be calculated. 

The amount of elevation of the boiling temperature is calculated using an equation similar to that used for freezing point depressions. As before, the concentration of the solute is in molality, but the constant, Kb, specifically applies to boiling points. 

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. 

Thus the molality, m, of a solution may be determined from the depression of freezing point. If the weight of solute, w, dissolved in 1 kg of solvent is known the molecular mass of the solute may be calculated from the relation M2 = 1000w/m. K iis is called the freezing-point-depression constant for the solvent. It may be readily calculated if the relevant properties of the solvent are known (Table 6.1), thus enabling the freezing-point equation to be used in a convenient form. 

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. 

The observed melting point of camphor is often low. Look up the molal freezing-point-de-pression constant K for camphor and calculate the expected depression of the melting point of a quantity of camphor that contains 0.5 molal impurity. Hint Look in a general chemistry book under "freezing-point depression" or "colligative properties of solutions."... 

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