75.2 g of C6H5OH (Phenol) is dissolved in a solvent of Kf = 14 , If the depression in freezing point is 7K then find the % of phenol that dimerises.

According to the concept of freezing point depression whenever a solute is added to a pure solvent, the freezing point of the solvent (which is now a solution) decreases. For example, when NaCl is dissolved in pure water, the freezing point of water is reduced from 0oC to – 21.1oC(Max).

an unknown solvent in which we are adding 75.2 g of Phenol. The Kf of the solvent is 14

The depression given to us is 7K, and we have to find the % of phenol that dimerises.

The equilibrium that must have established between different phenol molecules is given:

2C6H5OH ⇆ (C6H5OH)2

The molecular mass of phenol is 94.11g/mol

. So, the amount of Phenol added in solvent in moles will be

Given Mass/Molecular Mass = 75.2/94.11 = 0.8 moles

If α is the degree of dissociation, then the van’t hoff factor I will be (2−α/2)

Also, the depression in freezing point is given by KfXmoles Xi

Which will be, Depression in freezing point =7 = 14 × 0.8(2−α/2)

Solving this we will get α = 0.75, which means 75% of phenol is dimerises.

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