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The molar mass calculated through colligative properties is sometimes different from that of…
The molar mass calculated through colligative properties is sometimes different from that of experimentally determined molecular mass and is known as abnormal molecular mass. This difference is due to the solute particles that undergo association or dissociation.
Consider the two cases given below and answer the questions that follow.
Case (A): The freezing point of a solution containing $5\cdot85\text{ g}$ of $\text{NaCl}$ in $100\text{ g}$ of water is $-3\cdot348^\circ\text{C}$. ($K_f\text{ of water} = 1\cdot86\text{ K kg mol}^{-1}$, molecular mass of $\text{NaCl} = 58\cdot5$)
Case (B): The freezing point of benzene solution decreases by $0\cdot45^\circ\text{C}$ when $0\cdot2\text{ g}$ of acetic acid is added to $20\text{ g}$ of benzene. ($K_f\text{ of benzene} = 5\cdot12\text{ K kg mol}^{-1}$, molecular mass of acetic acid = 60)
Answer the following questions.
(i)[1.0]
Calculate the abnormal molecular mass of solute in both the cases.
(ii)[1.0]
Inferring the value of van’t Hoff factor in both the cases, find out which solution undergoes association and which solution undergoes dissociation.
(iii)[1.0]
Given below is the increasing order of depression in freezing point of water observed for equimolar concentrations of the compounds:
$\text{acetic acid} < \text{trichloro acetic acid} < \text{trifluoro acetic acid}$
Provide a reason to explain if the above ordered arrangement is correct or not.
Answer
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From ISC 2025 Specimen Chemistry Paper 1, question 13.