Molal freezing-point-depression

Impure substances have melting points that are very dependent upon the amount of impurity present. For a few substances this is quantified as the molal freezing point depression constant. The result is that melting points can be a very useful indicator of purification efforts. As long as each purification step in a process results in a higher melting point, the substance has been made more pure. This same concept allows the quality control chemist to have a very sensitive method for detecting impurities that is lower than anticipated. 

IQ = molal freezing-point depression constant Kb = molal boiling-point elevation constant Kf for water = 1.86 K kg mol-1 for water = 0.512 K kg mol-1 AT = iKf x molality ATb = iKb x molality n = MRT... 

Kf = molal freezing-point depression constant Kt,= molal boiling-point elevation constant A = absorbance a= molar absorptivity b = path length c= concentration Q = reaction quotient /= current (amperes) q= charge (coulombs) f= time (seconds)... 

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. 

For example, the observation that the freezing point of a 1 molal solution of acetone in sulfuric acid is depressed by twice the molal freezing-point depression constant of sulfuric acid is interpreted in terms of the reaction... 

A 0.01 molal solution of COH in H2S04 freezes at 10.09°C. The freezing point of pure H2S04 is 10.36°C, and the molal freezing-point depression constant is 6.81 °C. Explain. 

Molal Boiling-Point-Elevation Constants (Afb) and Molal Freezing-Point-Depression Constants (ACf) for Some Common Substances... 

PROBLEM 11.20 Assuming complete dissociation, what is the molality of an aqueous solution of KBr whose freezing point is —2.95°C The molal freezing-point-depression constant of water is given in Table 11.4. 

A solution prepared by dissolving 3.00 g of ascorbic acid (vitamin C, C HsOg) in 50.0 g of acetic acid has a freezing point that is depressed by AT = 1.33°C below that of pure acetic acid. What is the value of the molal freezing-point-depression constant for acetic acid ... 

Kf is called the molal freezing-point depression constant, since it is equal to the freezing-point depression predicted by Eq. (11) for a 1 molal solution. 

Molal freezing-point depression constant (K molal ) Kf 20.4 1.855... 

Molal freezing-point depression constant a constant characteristic of a particular solvent that gives the change in freezing point as a function of the solution molality used in molecular weight determinations. (17.5)... 

The Osmotic Coefficient.—Instead of calculating activity coefficients from freezing-point and other so-called osmotic measurements, the data may be used directly to test the validity of the Debye-Hiickel treatment. If 6 is the depression of the freezing point of a solution of molality m of an electrolyte which dissociates into v ions, and X is the molal freezing-point depression, viz., 1.858° for water, a quantity , called the osmotic coefficient, may be defined by the expression... 

The cryoscopic method was used in Hantzsch s original work 53, 54) and was later developed by Hammett and Deyrup 56) and by Gillespie et al. 48). Sulfuric acid freezes at 10.371 C and has a relatively large molal freezing-point depression or cryoscopic constant fc = 6.12 32). It is thus a convenient solvent for cryoscopic measurements provided that adequate precautions are taken to prevent absorption of water from the atmosphere. From freezing-point measurements the number, v, of moles of particles (ions and molecules) produced by one mole of any solute may be obtained. Some examples are given in Table IV. 

One mole of solute dissolved in 1000 g of solvent (to produce a 1 molal solution) depresses the freezing point by the molal freezing point depression constant (X/). The degree to which the freezing point of any solution is depressed (compared to that of the pure solvent) is directly proportional to the molality of the solute present. 

Glycol (C2H602) is the main component in antifreeze. What mass of glycol must be added to 10 liters of water to prevent freezing down to -18.6°C (The molal freezing point depression constant for water is 1.86°C kg/mol.)... 

Since Lo is the molar heat of fusion of the pure solvent at its fi-cczing point, at 1 atm., ijroMi/lOOOZ/o is exactly equivalent to the molal freezing point depression constant X, as defined by equation (36.12) hence equation... 

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. 

Pure copper melts at 1083°C. Its molal freezing point depression constant is 23°C/m. What will be the melting point of a brass made of 10% Zn and 90% Cu by mass ... 

K = molal freezing-point depression constant = molal boiling-point elevation constant... 

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