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9.SOLUTIONS eLearn.Punjab

This will produce 0.1 molal solution of each substance. Pure H2O has certain value of vapour
pressure at a given temperature. In these three solutions, the vapour pressures will be lowered.
The reason is that the molecules of a solute present upon the surface of a solution decrease the
evaporating capability. Apparently, it seems that sucrose solution should show the maximum
lowering of vapour pressure while urea should have the minimum lowering of vapour pressure.
The reality is that the lowering of vapour pressure in all these solutions will be same at a given
temperature. Actually, the number of particles of the solute in all the solutions are equal. We have
added 1/10th of Avogadro’s number of particles (6.02 x 1022). The lowering of vapour pressure
depends upon the number of solute particles and not upon their molar mass and structures. Well,
it should be kept in mind that these three solutes are non-volatile and non-electrolyte.
The boiling points of these solutions are higher than that of pure solvent. It is observed that
the boiling point elevation of these three solutions is 0.052 oC. Similarly, freezing points will be
depressed for these solutions and the value of depression in these three cases is 0.186°C. The
reason again is that the elevation of boiling point and the depression of freezing point depend
upon number of particles of solute.
Now, let us deduce the values of elevation of the boiling point and the depression of the
freezing point of water for 1 molal solutions. For that purpose, try to dissolve 60 g of urea. 180
g of glucose and 342 g of sugar separately in 1 kg of water. If, it is possible then the elevation of
boiling point and depression of freezing point of water will be 0.52 oC and 1.86 oC, respectively. All
the three solutions will boil at 100.52 °C and freeze at -1.86 oC. These values of elevation of boiling
point and depression of freezing point are called molal boiling point constants and molal freezing
point constants of H2O denoted by Kb and Kf respectively.
These are also named as ebullioscopic and cryoscopic constants, respectively. These constants
depend upon the nature of solvent and not upon the nature of solute. Following Table (9.3) give the
values of Kb and Kf for some common solvents.

Table (9.3) Kb and Kf values for some solvents

Solvent B.P.(0C) Kb(0C/m) F.P.(0C) Kf(0C/m)
100 0.52 0 1.86
H2O 34.4
Ether 2.16 -116.3 1.79

Aceticacid 118 3.07 17 3.90

Ethanol 79 1.75 -114.5 1.99

Benzene 80 2.70 5.5 5.10

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