CBSE 2025 · Region 5 · Set 1 · Q30 · 4 marks

Werner's coordination theory in $\displaystyle 1893$ was the first attempt to explain the bonding in coordination complexes. It must be remembered that this theory was put forward before the electron had been discovered by J.J. Thomson in $\displaystyle 1897$, and before the electronic theory of valency. Werner did not have any of the modern instrumental techniques and all his studies were made using simple experimental techniques. Werner was able to explain the nature of bonding in complexes and he concluded that in complexes, the metal shows two different sorts of valency : primary and secondary. Primary valences are normally ionisable whereas secondary valences are non ionisable. Answer the following questions :

(a)

One mole of $\displaystyle \mathrm{CrCl}_{3} \cdot 4 \mathrm{H}_{2} \mathrm{O}$ precipitates one mole of AgCl when treated with excess of $\displaystyle \mathrm{AgNO}_{3}$ solution. Write (i) the structural formula of the complex, and (ii) the secondary valency of Cr.

(b)

What is the difference between a complex and a double salt ?

(c)

(i)OR(c)

Arrange the following complexes in the increasing order of conductivity of their solution : $\displaystyle \left[\mathrm{Cr}\left(\mathrm{NH}_{3}\right)_{3} \mathrm{Cl}_{3}\right], \quad\left[\mathrm{Cr}\left(\mathrm{NH}_{3}\right)_{6}\right] \mathrm{Cl}_{3}, \quad\left[\mathrm{Cr}\left(\mathrm{NH}_{3}\right)_{5} \mathrm{Cl}^{-} \mathrm{Cl}_{2}\right.$

CBSE Class 12 Chemistry past-paper question from the 2025board exam, with the answer as CBSE’s own marking scheme gives it. Where our answers come from.