Question

Pure water as well as separate equimolal and dilute aqueous solutions of $NaCl , K _{3}\left[ Fe ( CN )_{6}\right]$ and $K _{4}\left[ Fe ( CN )_{6}\right]$ represented respectively as $I$ II, III and IV are available. Which of the following statements is correct, assuming $100 \%$ dissociation of all solutes?

• A. I < II < III < IV is the increasing order of freezing points
• B. IV < III < II < I is the increasing order of freezing points
• C. IV < III < II < I is the increasing order of boiling points
• D. IV < III < II < I is the increasing order of van't Hoff factors

Answer 1

Answer: (B)

$\Delta T_{f}=i \times K_{f} \times m . i$ values of $NaCl , K _{3}\left[ Fe ( CN )_{6}\right]$ and $K _{4}\left[ Fe ( CN )_{6}\right]$ are respectively $2,4$ and $5$.
Hence, $K _{4}\left[ Fe ( CN )_{6}\right]$ shows the largest depression and the lowest freezing point.
Water has the highest freezing point of $0^{\circ} C$.
Also $\Delta T_{b}=i \times$ molality.
IV has the highest $\Delta T_{b}$ and the highest boiling point.
The increasing order of boiling points is $I < II < III < IV$