What is the percent by mass of iodine needed to reduce the freezing point of benzene to `3.5^(@)"C"` ? The freezing point and Cryoscopic constant of pure benzene are `5.5^(@)"C"` and `5.12K//m` respectively.

Explanation

`Delta"T"_(f)="T"_(f)^(0)-"T"_(f)="K"_(f)."m"`
`5.5^(@)"C"-3.5^(@)"C"=5.12xxm`
`m=2/5.12=0.39" molal"`
`therefore` Mass of iodine needed for `1000` g of benzene `=mxx` molecular mass of iodine `I_(2)`
`=0.39` mol/kg `xx254` g/mol `=99.06` g/kg
`therefore` `1000` g `+ 99.06` g solution contain `99.06` g `I_(2)`
`100` g solution contains `99.06` g`xx100/1099.06` g `=9.01%`