Solutions and Colligative Properties - Result Question 68
####39. If the freezing point of a 0.01 molal aqueous solution of a cobalt (III) chloride-ammonia complex (which behaves as a strong electrolyte) is $-0.0558^{\circ} C$, the number of chloride(s) in the coordination sphere of the complex is
$$ \left[K _f \text { of water }=1.86 K kg mol^{-1}\right] $$
(2015 Adv.)
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Solution:
- $1+\Delta T _f=i K _f m$
$\Delta T _f=0-\left(-0.0558^{\circ} C\right)$
$$ =0.0558^{\circ} C $$
$\Rightarrow i$ (vant Hoff’s factor) $=\frac{0.0558}{1.86 \times 0.01}=3$
This indicates that complex upon ionisation produces three ions as:
$\left[Co\left(NH _3\right) _5 Cl\right] Cl _2 \rightarrow\left[Co\left(NH _3\right) _5 Cl\right]^{2+}(a q)+2 Cl^{-}(a q)$
Thus, only one $Cl$ is inside the coordination sphere.
