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(A) The relative lowering of vapour pressure is given by $\frac{P^o - P}{P^o} = \chi_{solute} = \frac{n}{n + N}$.Since the amount of solvent (water) i

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Same as that of $2^{nd}$ solution

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(A) The relative lowering of vapour pressure is given by $\frac{P^o - P}{P^o} = \chi_{solute} = \frac{n}{n + N}$.Since the amount of solvent (water) is the same $(1 \ L)$ and the amount of solute is small,the mole fraction is directly proportional to the number of moles of solute $(n)$.For the first solution,moles of urea $= \frac{12 \ g}{60 \ g/mol} = 0.2 \ mol$.For the second solution,moles of cane sugar $= \frac{68.4 \ g}{342 \ g/mol} = 0.2 \ mol$.Since the number of moles of solute is the same in both solutions,the lowering of vapour pressure will be the same.

$A$ solution is obtained by dissolving $12 \ g$ of urea (mol.wt. $60$) in a litre of water. Another solution is obtained by dissolving $68.4 \ g$ of cane sugar (mol.wt. $342$) in a litre of water at the same temperature. The lowering of vapour pressure in the first solution is

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