1.

The vapour pressure of a 5% aqueous solution of a non-volatile organic substance at 373 K. Is 745 mm. Calculate the molecular mass of the solute.

Answer»

Solution :A 5% aqueous solution means 5 g of solute in 100 g of solution.
Mass of solution (WATER)=100-5=95g
Vapourt pressure of solution at 373 K = 745 MM
Vapour pressure of water at 373 K 760 mm
According to Raoult's Law, `=(p_(A)^(@)-P_(S))/P_(S)=n_(B)/n_(A)=W_(B)/M_(B)xxM_(A)/W_(A)`
`((760mm-745mm))/((745mm))=(%5gxx(18g MOL^(-1)))/(M_(B)XX(95g))`
`15/745=((5g)xx(18gmol^(-1)))/(M_(B)xx(95g))`
`M_(B)=((5g)xx(18gmol^(-1))xx745)/((95g)xx15)=47 g mol^(-1)`


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