Department of Pre-University Education, KarnatakaPUC Karnataka Science Class 12

# 3.9 g of benzoic acid dissolved in 49 g of benzene shows a depression in freezing point of 1.62 K. Calculate the van't Hoff factor and predict the nature of solute (associated or dissociated). - Chemistry

Numerical

3.9 g of benzoic acid dissolved in 49 g of benzene shows a depression in freezing point of 1.62 K. Calculate the van't Hoff factor and predict the nature of solute (associated or dissociated).

(Given : Molar mass of benzoic acid = 122 g mol−1, Kf for benzene = 4.9 K kg mol−1)

#### Solution

We know that the depression in freezing point is given by

DeltaT_

Here,

van't Hoff factor = i

Depression in freezing point, ΔTf=1.62 K

Kf for benzene=4.9 K kg mol1

Mass of benzoic acid, ws=3.9 g

Mass of benzene, W=49 g

Molar mass of benzoic acid, Ms=122 g mol1

Substituting the values, we get

1.62 = (ixx4.9xx(3.9xx1000))/(122xx49)

rArri=(1.62xx122xx49)/(4.9xx3.9xx1000)

= 0.51

As the value of 1, benzoic acid is an associated solute.

Concept: Abnormal Molar Masses
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