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At 25^(@)C, the vapour presure of pure water is 23.76 mm of Hg and that of an aueous dilute solution of urea is 22.98 mm of Hg. What is the molalitty of the solution ?

Answer»


Solution :ACCORDING to Raoult's Law, `P=P_(A)^(@)X_(A)`
`22.98 mm Hg=23.76 mm HgxxX_(A) or X_(A)=((22.98 mm))/(23.76 mm)=0.967`
`"Mole FRACTION of solute"(X_(B))=1-X_(A)=1-0.967=0.033`
`"MASS of water"=(0.976 MOL)xx(18 g mol^(-1))=17.406 g`
17.405 g of water contain solute = 0.033 mole
`"1000 g of water contain solute "=((0.033mol))/((17.406 g))xx1000 g = 1.896 mol`
`therefore "Molality of solution"=1.896 m.`


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