Question

The freezing point order of the solution of glucose is?
A) \[10\% > 3\% > 2\% > 1\% \]
B) \[1\% > 2\% > 3\% > 10\% \]
C) \[1\% > 3\% > 10\% > 2\% \]
D) \[10\% > 1\% > 3\% > 2\% \]

Answer 1

Hint: We need to know the freezing point, temperature at which a liquid becomes a solid. As with the melting point, increased pressure usually raises the freezing point. The freezing point is lower than the melting point in the case of mixtures and for certain organic compounds such as fats.

Complete answer:
We must remember that the freezing point depression \[\Delta T = KF \times m\] where \[KF\] is the molal freezing point depression constant and m is the molality of the solute. Freezing point depression is a colligative property observed in solutions that results from the introduction of solute molecules to a solvent. The freezing points of solutions are all lower than that of the pure solvent and are directly proportional to the molality of the solute.
Let’s solve the question: We can take the mass of solvent to be $100g$ so $10\% $ solution of glucose means $10g$ of glucose in $100g$ of water. Similarly, \[3\% \] solution of glucose means $30g$of glucose in $100g$ of water and \[2\% \] solution of glucose means $20g$ of glucose in $100g$ of water. The number of moles is directly proportional to molality, greater the number of moles, greater will be the molality therefore, \[10\% \] solution will have the greater molality than $3\% $ followed by $2\% $. This means that change in freezing point will be greatest for \[10\% \] than \[3\% \] than \[2\% \] and than \[1\% \].Greater the change less will the freezing point as there is depression in freezing point on addition of solute.

Thus the correct option is B i.e. \[1\% > 2\% > 3\% > 10\% \].

Note:
We have to know that ordinarily, the freezing point of water and melting point is $0^\circ C$ or $32^\circ F$. The temperature may be lower if supercooling occurs or if there are impurities present in the water which could cause freezing point depression to occur.