Question

A complex compound of $Co ^{3+}$ with molecular formula $CoCl _{ x } \cdot yNH _{3}$ gives a total of $3$ ions when dissolved in water. How many $Cl ^{-}$ ions satisfy both primary and secondary valencies in this complex?

• A. 3
• B. 1
• C. 4
• D. zero

Answer 1

Answer: (B)

As cobalt is present as $Co ^{3+}$ and co-ordination number of cobalt is $6$, the molecular formula of compound should be $CoCl _{3} \cdot yNH _{3}$.
Now, as it gives a total of $3$ ions when dissolved in water, its structural formula must be $\left[ CoCl \left( NH _{3}\right)_{5}\right] Cl _{2}$
$\left[ CoCl \left( NH _{3}\right)_{5}\right] Cl _{2} \rightleftharpoons\left[ CoCl \left( NH _{3}\right)_{5}\right]^{2+}+2 Cl ^{-}$
Thus only one $Cl ^{-}$ ion is satisfying both primary and secondary valency of $Co ^{3+}$ in this compound.