A solution of lactose containing 8.45 g of lactose in 100 g of water has a vapour pressure of 4.5 mm of Hg at ` 0^(@) C ` . If the vapour pressure of pure water is ` 4.579 mm ` of Hg . Calculate the molar mass of lactose.

Explanation

`P^(@) = 4.579 mm "of" Hg`
` P= 4.5 mm "of Hg" `
`W_2 -8.4 g `
` W_1 =100g `
` M_1 =18`
` P^(@) -P " "= 4.579-4.5 =0.079`
` (P^(@) -P)/( P) =( 0.079)/( 4.579) `
` (P^(@) - P)/( P) =(8.45)/( M_2) xx (18)/( 100) `
` 0.0712" "= ( 152.1)/( 0.0172) xx ( 152.1)/( 100) =88.43 g//mol `