. ) Where, M B =molar mass of the solute Therefore, m w M w A ............... ( . ) and ∆ T w M w b b A ...............
( . ) Molar mass can be calculated by using ( . ) M w T w b b A ∆ ............... ( .
) Table . Molal boiling point elevation constant ‘K b ’ for some solvents S. No. Solvent T b ° (K) K b (K kg mol - ) .
Water . . . Ethanol .
Ether . . . Carbon tetrachloride .
. . Carbon disulphide . .
Cyclohexane . . Example Problem . g of an unknown substance is dissolved in g solvent.
If the elevation of boiling point is . K and molal elevation constant is . K Kg mol - then, calculate the molar mass of unknown substance ΔT b = K b m = K b x W x / M x W M = K b x W x / ΔT b x W = . x .
g of Sulphur is dissolved in 100g of carbon disulphide. The solution boils at . K . What is the molar mass of Sulphur in solution The boiling point of CS is .
450K. Given that K b for CS = . K Kg mol - Evaluate Yourself Unit Unit - - - - Depression in freezing point Freezing point of a substance is another important physical property like boiling point. Freezing point is defined as “the temperature at which the solid and the liquid states of the substance have the same vapour pressure”.
At freezing point, the solid and liquid phases of the substance are in equilibrium. For example, the freezing point of water is ˚C. At this temperature the