1.

Freezing point of urea solution is -0.6^(@)C. How much urea (M.W. = 60 g/mole) is required to dissolve in 3 kg water (K_(f) = 1.5^(@)Ckg mol^(-1))

Answer»

3.6 g
2.4 g
7.2 g
6.0 g

Solution :`DELTA T_(F)=K_(f)xx(1000xxW_(A))/(M_(A)xxW_(B))`
[where `W_(A)` = WEIGHT of SOLUTE, `W_(B)` = weight of solvent, `M_(A)` = molecular weight of solute]
or, `0-(-0.6)=(1.5xx1000xxW_(A))/(60xx3000)`
or, `W_(A)=(60xx3000xx0.6)/(1.5xx1000)=72g`
If we consider the mass of water given to 300 gram `0.6^(@)C=(1000g kg^(-1)xx1.5^(@)Ckg mol^(-1)xx " weight of urea")/(60g mol^(-1)xx300 g)`
`therefore` Weight of urea = 7.2 g.


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