The molal freezing point depression constant for benzene `(C_(6)H_(6))` is 4.90 K kg `mol^(-1).` Selenium exists as a polymer of the type Sex. When 3.26 g of selenium is dissolved in 226 g of benzene, the observed freezing point is `0.112^(@)C` lower than that of pure benzene. The molecular formula of selenium is:
(Atomic mass of Se `=78.8 g mol ^(-1))`

Options

A. `Se _(4)`
B. `Se _(2)`
C. `Se _(6)`
D. `Se _(8)`

Answer

D

Explanation

Calculateion of molar mass of sodium (Sex).
`M _(B) = ( K _(f) xx W _(B))/( Delta T _(f) xx W _(A))`
Mass of selenium `(W _(B)) = 3. 26 g`
mass of benzene
`(W _(A)) = 226 g = 9. 226 kg `
Depression in freezing point
`(Delta T _(f)) = 0. 112 ^(@)C`
`= 0. 112 K`
Molar depression constant
`(K _(f)) = 4. 9 K kg mol ^(-1)`
`M _(B) = (( 4. 9 kg mol ^(-1)) xx ( 3. 26 g ))/( ( 0. 112 K ) xx ( 0. 228 kg )) = 6 31 . 08 g mol ^(-1)`
The gram atomci mass of selenium
`= 7.8 g mol ^(-1)`
In Sex, `x ( 78.8 g mol ^(-1)) = 631. 08 g mol ^(-1)`
`x = ( 631 . 08 g "mol" ^(-1))/( 78.8 g " mol" ^(-1)) = 8`
`therefore` Molecular formula of selenium `= Se _(8)`