The vapour pressure of benzene at`80^(@)C` is lowered by `10mm` by dissoving `2g`of a non-volatile substance in`78g`of benzene .The vapour pressure of pure benzene at `80^(@)C`is `750mm`.The molecular weight of the substance will be:

Options

A. 150
B. 1050
C. 1500
D. 1550

Answer

A

Explanation

If `"P"^(@)` is the vapour pressure of pure benzene and P the vapour pressure of solution, w the weight of non-volatile substance of molecular mass 'm' and w the weight of solvent benzene with molecular mass M,
`("P"^(@)-"P")/"P"^(@)=("w"//"m")/("W"//"M")=("w""M")/("W""m")" substituting the values"`
`10/750=(2xx78)/(78xx"m")`
`0.0133=156/ (78xx"m")` Or `"m"=156/(78xx0.0133)=150.37~150`
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