Question

When 1 mol $CrC{{l}_{3}}.6{{H}_{2}}O$ is treated with excess of $AgN{{O}_{3}}$, 3 mol of are obtained. The formula of the complex is:
A. $\left[ CrC{{l}_{3}}{{\left( {{H}_{2}}O \right)}_{3}} \right].3{{H}_{2}}O$
B. $\left[ CrC{{l}_{2}}{{\left( {{H}_{2}}O \right)}_{4}} \right]Cl.2{{H}_{2}}O$
C. $\left[ CrCl{{\left( {{H}_{2}}O \right)}_{5}} \right]C{{l}_{2}}.{{H}_{2}}O$
D. $\left[ Cr{{\left( {{H}_{2}}O \right)}_{6}} \right]C{{l}_{3}}$

Answer 1

The primary valence or primary valences in chemistry in a given coordination compound is defined as the number of negative ions needed to satisfy the charge on the central metal ion. Primary valence is the number of necessarily negatively charged ions which satisfy the charge on the metal ion and it is written outside the coordination sphere, that is it makes up the ionization sphere. Primary valency is ionisable valency and non-directional while secondary valence are those which satisfy the coordination number of central metal ions and it is written in coordination sphere. It is non-ionisable and directional.

Complete step by step answer:
When 1 mol $CrC{{l}_{3}}.6{{H}_{2}}O$ is treated with excess of $AgN{{O}_{3}}$​, 3 mol of $AgCl$ are obtained. It means that when $CrC{{l}_{3}}.6{{H}_{2}}O$ molecule dissociates in aqueous solution, all three chloride ions come in solution as they are the primary valances. The formula of the complex is $\left[ Cr{{\left( {{H}_{2}}O \right)}_{6}} \right]C{{l}_{3}}$​. The IUPAC name for the given compound is Hexaaquachromium(III)chloride. The reaction taking place in given below:
$\left[ Cr{{\left( {{H}_{2}}O \right)}_{6}} \right]C{{l}_{3}}\to {{\left[ Cr{{\left( {{H}_{2}}O \right)}_{6}} \right]}^{3+}}+3C{{l}^{-}}$
$3C{{l}^{-}}+3AgN{{O}_{3}}\to 3AgCl\downarrow +3NO_{3}^{-}$
Hence, the correct option is option D which is $\left[ Cr{{\left( {{H}_{2}}O \right)}_{6}} \right]C{{l}_{3}}$.

Note:
Alfred Werner in 1898 proposed Werner's theory explaining the structure of coordination compounds. Werner's Experiment: By mixing $AgN{{O}_{3}}$ (silver nitrate with $CoC{{l}_{3}}.N{{H}_{3}}$, all three chloride ions got converted to $AgCl$silver chloride. However, when $AgN{{O}_{3}}$ wa.s mixed with $CrC{{l}_{3}}.6{{H}_{2}}O$, three moles of $AgCl$ were formed. The primary valency in coordination compound relates to the oxidation state and the secondary valency relates to the coordinate number. According to Werner’s theory the number of secondary valences is fixed for every metal atom.