Question

The freezing point of benzene decreases by `0.45^(@)C` when 0.2 g of acetic acid is added to 20 g of benzene. If acetic acid associates of form a dimer in benzene, percentage association of acetic acid in benzene will be `(K_(f)"for benzene"=5.12 "K kg " mol^(-1))`.
A. 0.746
B. 0.946
C. 0.646
D. 0.804

Answer 1

Correct Answer - b
`"Molality (m)"=("No.of moles of "CH_(3)COOH)/("Mass of benzene in kg")`
`((0.2))/((60 g mol^(-1))xx(0.02kg))`
`=0.167 mol kg^(-1)=0.167m`
`DeltaT_(f)=ixxK_(f)xxm or i=(DeltaT_(f))/(K_(f)xxm)`
`i=((0.45K))/((5.12 "K kg mol"^(-1))xx(0.167"mol kg "^(-1)))=0.526`
`alpha=((i-1))/((n-1))=((0-526-1))/((0.5-1))`
`=((-0474))/((-0.5))=0.948 %`