Molal freezing point elevation constant

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)... 

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

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

The proportionality constant K( is the molal freezing-point-depression constant, analogous to for boiling point elevation. Note that because the solution freezes at a lower temperature than does the pure solvent, the value of AT is negative. 

Table 11.5 Molal Boiling-Point Elevation Constants Kb) and Freezing-Point Depression Constants (Kf) for Several Solvents...

Use the data in Table 12.4 to evaluate the molal freezing-point depression constant and the molal boiling-point elevation constant for H2O at a pressure of 1 bar. 

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. 

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. 

Vapor-pressure lowering, boiling-point elevation, and freezing-point depression are very similar thermodynamically. For example, the increase in boiling point ATh is interpreted thermodynamically by using the boiling-point elevation constant Kb to obtain the molality of the solution, as stated in the equation... 

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 ... 

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. 

This equation may be used to determine molecular masses in exactly the same manner as the freezing-point equation. It may be simplified as before to give AT- Kvapm where m is the molality, AT is the elevation of the boiling point, and is the boiling-point-elevation constant ... 

TABLE 12.2 Molal Boiling-Point Elevation and Freezing-Point Depression Constants of Several Common Liquids ... 

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