Question

when 2g of benzoic acid is dissolved in 25g of benzene ,the experimentally determined molar mass is always greater then rurle value.give reason

Answer 1

When 2 g of C6H5COOH dissolved in 25g of benzene , the molar mass that is determined experimentally is constantly more prominent and greater than the real value due to the reason that C6H5COOH will dimerize.

It can be proved mathematically,

weight of C6H5COOH = w1 = 2 g

weight of C6H6 =  W2 = 25 g  

change in freezing temperature = ∆T = 1.62

co-efficient of freezing point= Kf = 4.9

As we know that,

ΔT = (1000 × Kf  × w1) / (M × W2)

M= molar mass of product  

M = 241.98 grams per mole  

Theoretically,  

nC6H5COOH -----------> (C6H5COOH)n

after associating = i = 1 - α + α/n

For dimerization ,

n = 2

i =  1 - α + α/2 = 1 - α/2

Van Hoff's theory,  

M theoretical/ M experimental = 1 - α/2

M theoretical = 122 grams per mole

122 / 241.98 = 1 -α/2

α = 0.992

Association constant is less than one. So determined M will always be greater than true value.