Question

For $1$ molal aqueous solution of the following compounds, which one will show the highest freezing point ?

• A. $[Co(H_2O)6]Cl_3$
• B. $[Co(H_2O)_5Cl]Cl_2.H_2O$
• C. $[Co(H_2O)_4Cl_2]Cl._2H_2O$
• D. $[Co(H_2O)_3Cl_3]._3H_2O$

Answer 1

Answer: (D)

$[Co(H_2O)_3Cl_3]._3H_2O$

Answer 2

The solution which shows maximum freezing point must have minimum number of solute particles.

(1) $\left[ Co \left( H _{2} O \right)_{6}\right] Cl _{3} \rightarrow\left[ Co \left( H _{2} O \right)_{6}\right]^{3+}+3 Cl ^{-}, i =4$

(2) $\left[ Co \left( H _{2} O \right)_{5} Cl \right] Cl _{2} \cdot H _{2} O \rightarrow\left[ Co \left( H _{2} O \right)_{5} Cl \right]^{2+}+2 Cl ^{-},i=3$

(3) $\left[ Co \left( H _{2} O \right)_{4} Cl _{2}\right] Cl \cdot 2 H _{2} O \rightarrow\left[ Co \left( H _{2} O \right)_{4} Cl _{2}\right]^{+}+ Cl,i=2$

(4) $\left[ Co \left( H _{2} O \right)_{3} Cl _{3}\right] \cdot 3 H _{2} O \rightarrow\left[ Co \left( H _{2} O \right)_{3} Cl _{3}\right], i =1$

So, solution of $1$ molal $\left[ Co \left( H _{2} O \right)_{3} Cl _{3}\right] \cdot 3 H _{2} O$ will have minimum number of particles in aqueous state.