BOILING-POINT ELEVATION

When a nonvolatile solute is dissolved in water or another suitable solvent, the normal boiling point of the solvent is always raised. The amount of elevation in temperature, AT, is given by the equation

where i = the van’t Hoff factor (discussed above) m = the molality of the solution

Kb = the ebullioscopic or boiling-point-elevation constant, characteristic of water (or other solvent) in °Ckg/mol. See Table 1.5 which lists Kb (and Kf ) values for common solvents.

Example 1.17

What is the boiling point of a solution made by dissolving 70 g of NaCl in 300 g of water? The Kb for water is 0.512°Ckg/mol.

Solution

Use equation 1.12: AT = iKb m, and note that i = 2 for NaCl. Then, molality,

Table 1.5. Cryoscopic and ebullioscopic constants for common solvents

Substance

Freezing Point (°C)

Cryoscopic

Constant

(K‘kg/mol)

Boiling

Point

(°C)

Ebullioscopic

Constant

(K‘kg/mol)

Acetic acid

16.6

3.90

118.1

3.07

Benzene

5.5

5.12

80.1

2.53

Camphor

179.8

39.7

204.0

5.95

Cyclohexane

6.4

20.2

80.74

2.79

Substance

Freezing Point (°C)

Cryoscopic Constant (Ю kg/mol)

Boiling

Point

(°C)

Ebullioscopic

Constant

(K‘kg/mol)

Diethyl ether

-114.3

1.79

34.5

2.16

Ethanol

-114.6

1.99

78.4

1.19

Water

0.0

1 .86

100.0

0.512

 
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