Boiling point elevation constants

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. 

B. In the equation At = i m - Kb, where At is the boiling-point elevation, m is the molality of the solution, and Kb is the boiling-point-elevation constant for water, i (the van t Hoff factor) would be expected to be 4 if H3B03 were completely ionized. According to data provided, i is about 1.5. Therefore, H3B03 must have a relatively low Ka. 

C. The boiling-point-elevation constant of 0.512°C kg/mole would be expected to raise the B.P. 0.0256°C for a 0.05 m solution when i = 1. The data show that the boiling-point elevation is 0.0255°C. This agrees with the theory. Therefore, C6Hi206 does not dissociate. With few or no ions in solution, poor electrical conductivity is expected. This is supported by the evidence in the table. 

In this equation, ATb is the number of degrees that the boiling point has been elevated (the difference between the boiling point of the pure solvent and the solution), Kb is the boiling-point elevation constant, m is the molality of the solute, and i is again the van t Hoff factor. 

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... 

What is the molality of an aqueous glucose solution if the boiling point of the solution at 1 atm pressure is 101.27°C The molal boiling-point-elevation constant for water is given in Table 11.4. 

A solution prepared by dissolving 5.00 g of aspirin, C9H8O4, in 215 g of chloroform has a normal boiling point that is elevated by AT = 0.47°C over that of pure chloroform. What is the value of the molal boiling-point-elevation constant for chloroform ... 

A solution of citric acid, C HsC, in 50.0 g of acetic acid has a boiling point elevation of AT = 1.76°C. What is the molality of the solution if the molal boiling-point-elevation constant for acetic acid is Kb = 3.07 (°Okg)/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... 

TABLE 2. Freezing-point depression and boiling-point elevation constants... 

Show that for a substance obeying Trouton s rule, the boiling-point elevation constant is proportional to the boiling point. 

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 ... 

Freezing Point Depression and Boiling Point Elevation Constants for Various Solvents... 

Solutions containing nonelectrolyte nonvolatile solutes have higher boiling points than the pure solvent. The boiling point elevation (ATb) is directly proportional to the solvent s boiling point elevation constant (Ky) times the molality (m) of the solute in moles per kg of solvent ... 

The boiling-point elevation constant for water is 0.512°C/m (see Table 15.3). Thus,... 

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

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