What weight of the non -volatile solute, urea needs to be dissolved in 100g of water, in order to decrease the vapour pressure of water by 25%? What will be the molality of the solution?

Explanation

Using Raoult's law in the following form,
`(p_(0)-p_(s))/(p_(s))=(wM)/(Wm)`
If`p_(0)=100` mm, then `p_(s)=75` mm
`(100-75)/(75)=(wxx18)/(100xx60)`
`w=111.1`
Molality `= (wxx1000)/(mxxW)=(111.1xx1000)/(60xx100)=18.52m`.