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The vapour pressure of acetone at 20^(@)...

The vapour pressure of acetone at `20^(@)C` is 185 torr. When 1.2 g of a non-volatile substance was dissolved in 100 g of acetone at `20^(@)C` , its vapour pressure was 183 torr. The molar mass `( " g mol"^(-1))` of the substance is :

A

32

B

64

C

128

D

488

Text Solution

Verified by Experts

The correct Answer is:
B
Vap. pressure of pure acetone, `p_(A)^(@)`
`=185` torr
Vap. pressure of solution, `p=183` torr
Molar mass of solvent, `M_(A)=58 "g mol"^(-1)`,
`w_(B)=1.2 g, w_(A)=100 g`
`(p_(A)^(@)-p)/(p_(A)^(@))=x_(B)=(n_(B))/(n_(A))`
`(p_(A)^(@)-p)/(p_(A)^(@))=(w_(B))/(M_(B))xx(M_(A))/(w_(A))`
`(185-183)/(185)=(1.2)/(M_(B))xx(58)/(100)`
`(2)/(185)=(1.2xx58)/(M_(B)xx100)`
`:.M_(B)=(1.2xx58xx185)/(2xx100)=64.38`
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