Water boiling point elevation constant

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. 

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... 

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. 

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

The molal boiling point elevation constant, S), is the difference in boiling points between aim nonvolatile, nonelectrolyte solution and a pure solvent. It is expressed in units of °C/w and varies for different solvents. Values of A j, for several common solvents are found in Table 15-4. Note that water s A j, value is 0.512°C/w. This means that a m aqueous solution containing a nonvolatile, nonelectrolyte solute boils at 100.512°C, a temperature 0.512°C higher than pure water s boiling point of 100.0°C. 

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 freezing point depression constant for water is 1,86°C/m, and the boiling point elevation constant is 0.52°C/m. 

Table 12.2 lists values of A), for several common solvents. Using the boiling-point elevation constant for water and Equation (12.6), you can see that if the molality of an aqueous solution is 1.00 m, the boiling point will be 100.52°C. 

ATb is the increase in the boiling point, Kt, is the boiling point elevation constant (0.512°C kg/mol for water), and m is the molality of particles. (Tor molecular substances, the molality oif particles is the Scune as the molality of the substance for ionic compounds, you have to take into consideration the formation of ions and calculate the molality of the ion particles.) Solvents other than water have a different boiling point elevation constant ( ). 

EXAMPLE 16.1 Compute the boiling point elevation constant Kt for water. 

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 amounts of species a and b in this example (1 mol of solute, lOOOg of solvent, which produce a solute concentration of 1.00 molal) were chosen to match the common presentation of boiling point elevation in chemistry textbooks. They normally report the boiling-point elevation coefficient for various solvents. For water at one atmosphere the molal boiling-point elevation constant Ai 0.51°C it has different values for other solvents. It is common in chemistry books to write that as... 

Use Eq. 8.V to estimate the value of the boiling-point elevation constant Kb for water at its NBP, using the value (4/Zwater, molar/ water)NBP = 539.4 cal/g. Compare it to the value shown in Example 8.15. 

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