Molal freezing-point

Molal Freezing Point and Boiling Point Constants... 

Molal boiling point constant, 269,270t Molal freezing point constant, 269,270t Molality (m) A concentration unit defined as the number of moles of solute per kilogram of solvent, 259,261-262 Molar mass The mass of one mole of a substance, 55,68-68q alcohol, 591 alkane, 591... 

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. 

Old United States silver coins, which are 10 0% Cu in Ag, melt completely at 875 0°C, pure silver melts at 960 0°C What is the molal freezing-point constant for Ag9... 

A brass sample composed of 20.0% Zn and 80.0% Cu melts completely at 995.0°C pure Cu melts at 1084.0°C. What is the molal freezing-point constant for copper ... 

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 ... 

Kip is the molal freezing-point constant of the solvent. Like Kbp, Kip is a property of the solvent, independent of the nature of the solutes. 

The freezing point of pure camphor is 178,4°C and its molal freezing-point constant, kf, is 40.0° dm. Find the freezing point of a solution containing 1.50 g of a compound that has the molar mass (M) 125 g/mol dissolved in 35.0 g of camphor. 

The freezing point of a sample of naphthalene was found to be 80.6°C. When 0.512 g of a substance were dissolved in 7.03 g naphthalene, the solution s freezing point was 75.2°C. What is the molar mass of the solute (Molal freezing-point constant for naphthalene is 6.80°C kg/mol.)... 

Pure benzene freezes at 5.45°C. A solution containing 7.24g C2CI4H2 in 115.3 g benzene was observed to freeze at 3.55°C. From these data, calculate the molal freezing-point constant for benzene. 

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. 

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