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Phenol associaes in bensen to a certain ...

Phenol associaes in bensen to a certain extent to from a dimer. A solution containing `2.0xx10^(-2)` kg of phenol in 1.0 kg of benzene has a greezing point decreased by 0.69 K. What is the degree of association of phenol ? (`K_(f)` for benzene=5.12 K kg `mol^(-1)`).

Text Solution

Verified by Experts

The correct Answer is:
0.734
Step I. Calculation of observed molar mass of phenol
`M_(B)=(K_(f)xxW_(B))/(W_(A)xxDeltaT_(f))=((5.12" K kg mol"^(-1))xx(2.0xx10^(-2)kg))/((1.0kg)xx0.69K)`
`=14.84xx10^(-2)" kg mol"^(-1)=148.4" g mol"^(-1)`
Step II. Calculation of Van't Hoff factor
`"Van't Hoff factor (i)"=("Normal molar mass of phenol")/("Obsrved molar mass of phenol")`
`=((94" g mol"^(-1)))/((148.4" g mol"^(-1)))=0.633`
Step III. `"Calculation of degree of association of phenol "(alpha)`
`2C_(6)H_(5)OHhArr(C_(6)H_(5)OH)_(2)`
`alpha=(i-1)/(1/n-1)=((0.633-1))/((1/2-1))=((-0.367))/((-0.500))=0.734 or 73.4`%
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