Solvents boiling/freezing point constants

The proportionality constants in these equations, fcb and kf, are called the moled boiling point constant and the moled freezing point constant, respectively. Their magnitudes depend on the nature of the solvent (Table 10.2). Note that when the solvent is water,... 

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... 

Each solvent has its own characteristic freezing-point constant Kv and boiling-point constant Ab, the changes caused by 1 mole of solute in 1 kilogram of solvent. Selected constants are given in Table 21-1. 

Solvent Freezing point of pure solvent (°C) Freezing point depression constant, Kf /°C kg solvent V mol solute J Boiling point of pure solvent CO Boiling point elevation constant, Kf, /°C kg solvent V mol solute /... 

Boiling and Freezing Point Constants for Various Solvents... 

The proportionality constants, Kf and K, are, respectively, the molal freezing-point depression constant and the molal boiling-point elevation constant The freezing-and boiling-point constants are properties of the solvent, no matter what the solute may be. The freezing-point constant for water is 1.86°C/m, and the boiling-point constant is 0.52°C/m. 

The molal boiling-point and freezing-point constants are used to calculate boiling-point elevations and freezing-point depressions of solvents containing nonvolatile solutes. 

A = Kj- Cflj A 7b = Ki) Cflj We use molality in these equations because they describe temperature changes. The constant Zf is called the freezing point depression constant, and is called the boiling point elevation constant. These constants are different for different solvents but do not depend on the identity of the solutes. For water, Zf is 1.858 °C kg/mol and is 0.512 °C kg/mol. 

Organic solvent Boiling point, T CF) Freezing point. °C CF) Viscosity, cgs, 25T (77°F) Dielectric constant, 25T (77T)... 

Table 3.6 lists Kf and Kb for several solvents. In general, the higher the molar mass of the solvent, the larger the values of Kf and Kb. If the freezing point depression and boiling point elevation constants are known, the molecular weight of the dissolved solute, M2, can be determined ... 

TABLE 17.5 Molal Boiling-Point Elevation Constants (Kb) and Freezing-Point Depression Constants (Kf) for Several Solvents... 

A solution of 5.000 g of a solute in 100.0 g of water is heated slowly at a constant pressure of 1.00 atm and is observed to boil at 100.421°C. Estimate the molecular weight of the solute, the effective solvent vapor pressure at 25°C and the solution freezing point at 1 atm. The necessary properties of water can be found in Table B.I. 

The raising of the boiling point and the lowering of the freezing point are therefore proportional to the molal concentration. The constant of proportionality can be calculated from the latent heat of evaporation or fusion, and from the boiling or freezing point of the solvent. 

Boiling point elevation AT = mK[, (the constants have been tabulated) Freezing point depression AT = —mK (the constants have been tabulated) A solution in contact with its pure solvent across a semi-permeable membrane experiences an increase in pressure as pure solvent flows through the membrane into the solution. This osmotic pressure can be measured quite accurately, and through the equation ttV = nRT permits determination of the molecular weight of the solute. 

The van t Hoff factor (/) is an important factor in predicting the change in boiling point or freezing point of a solution after a solute has been added. The van t Hoff factor is symbolized by the lower-case letter /. It is a unitless constant directly associated with the degree of dissociation of the solute in the solvent ... 

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. 

The determination of molecular weights by means of the depression of the freezing point is based upon the same principle as that underlying the determination by means of the rise in boiling point. The same laws hold. The constants, however, for the various solvents are not the same as those used in calculating molecular weights by the boiling point method. 

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