Question

12.2 g of benzonic acid (molar mass = 122 ) in 100 g of benzene has a depression in freezing point `2.6^(@)`. If there is 100 percent polymersation, what is the nunber of molecues of benzoic acid in associated state ? (Given `K_(f)` for benzene =5.2 K kh `mol^(-1)`)

Answer 1

Correct Answer - Dimar
Observed molar mass of benzoic acid may be calculated as follows :
`M_(B)=(K_(f)xxW_(B))/(DeltaT_(f)xxW_(A))`
`W_(B)=12.2g, W_(A)=100g=0.1 kg, K_(f)=5.2" K kg mol"^(-1), DeltaT_(f)=2.6 K`
`M_(B)=((5.2" K kg mol"^(-1))xx(12.2 g))/((2.6 K)xx(0.1"kg"))=244.0" g mol"^(-1)`
`"On comparison, observed molar mass "(244" g mol"^(-1)) "of benzoic acid is twice its normal molar mass (122" g mol"^(-1))`.
This means that benzoic acid exists as a dimer in the associated state.