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12.2 g of benzoic acid is dissolved in (...

`12.2 g` of benzoic acid is dissolved in (i) `1 kg` acetone `(K_(b) = 1.9 K kg mol^(-1))` and (ii) 1 kg benzene `(K_(b) = 2.6 K kg mol^(-1)`.
The elevation of boiling points are `0.19^(@)C` and `0.13^(@) C`, respectively.
a. What are the molar masses of benzoic acid in the two solutions ?
, b. What are the structures of benzoic acid in the two solutions ?

Text Solution

Verified by Experts

Let M be the molar mass of benzoic acid in the solution. The molality of the solution is
`m=(n_B)/(g_A)=((12.2g)//M)/(1kg)`
Since `DeltaT_b=K_b`m we get:
Benzene solution `0.13 k=(2.6 K kg mol^(-1))[(12.2 g//M_("Benzoic Acid")/(1kg)]`
`M_("Benzoic Acid") = (2.6xx12.2)/(0.13) g mol ^(-1) = 244 g mol ^(-1)`
The actual molar mass of benzoic acid `(C_6H_5COOH)` is 122 g `mol^(-1)` . This means benzoic acid is present as a monomer in acetone solution while it dimerises in benzene solution. The structure of dimerised benzoic acid is `(to)`
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