A solution of lactose containing 8.45 g of lactose in 100g of water has a vapour pressure of 4.559 mm of Hg at `0^(@)C`. If the vapour pressure of pure water is 4.579 of Hg, calculate the molecular weight of lactose.

Explanation

Given: Weight of lactose `w = 8*45 g`.
Weight of water `W = 100 g`
Vapour pressure of pure water
`= 4*579 mm` of Hg
Vapour pressure of solution
`= 4*559 mm` of Hg.
Now, according to Raoult.s law.
`(DeltaP)/P = (wxxM)/(m xx W)`
`(4*579 - 4*559)/(4*579) = (8*45 xx18)/(mxx 100)`
`m=(8*45 xx 18 xx 4*579)/(0*02 xx 100)`
`= 348*23 ~~348`
Molecular mass of lactose is 348.