Q12 of 35 Page 1

The freezing point of benzene decreases by 2.12 K when 2.5 g of benzoic acid (C6H5COOH) is dissolved in 25 g of benzene. If benzoic acid forms a dimer in benzene, calculate the van’t Hoff factor and the percentage association of benzoic acid. (Kf for benzene = 5.12 K kg mol-1)

Given: ΔTf = 2.12K


Kf = 5.12 K kg mol-1


Weight of benzoic acid (solute) = 2.5g


Molar mass of benzoic acid, (Msolute) = 12×6+1×5+12+16+16+1 =122 g/mol


Weight of benzene (solvent) = 25g


First we will find out the van’t Hoff factor by applying the formula:


ΔTf = i × Kf × m


Where m =


∴ 2.12K = i × 5.12 K kg mol-1 ×


⇒ i = 0.505


Thus, the van’t Hoff factor of benzoic acid is 0.505


Now to calculate percentage association of benzoic acid, we apply the formula given below:


i= 1 -


where α is the degree of association


By putting the values in the formula, we get


α = 0.99


Thus, the percentage association of benzoic acid is 99%


More from this chapter

All 35 →
10

Identify the following:

(i) Oxoanion of chromium which is stable in acidic medium.


(ii) The lanthanoid element that exhibits +4 oxidation state.

 11

Give the IUPAC name of the product formed when:

(i) 2-Methyl-1-bromopropane is treated with sodium in the presence of dry ether.


(ii) 1- Methyl cyclohexene is treated with HI.


(iii) Chloroethane is treated with silver nitrite.

 13

Explain the following behaviours:

(i) Alcohols are more soluble in water than the hydrocarbons of comparable molecular masses.


(ii) Ortho-nitrophenol is more acidic than ortho-methoxyphenol.


(iii) Cumene is a better starting material for the preparation of phenol.

 14

The rate constant for a first order reaction is 60 s-1. How much time will it take to reduce 1g of the reactant to 0.0625 g?