Question:

On treatment of $100\, mL$ of $0.1\, M$ solution of $CoCl _{3} \cdot 6 H _{2} O$ with excess $AgNO _{3} ; 1.2 \times 10^{22}$ ions are precipitated. The complex is

Updated On: Jul 6, 2024
  • $\left[ Co \left( H _{2} O \right)_{6}\right] Cl _{3}$
  • $\left[ Co \left( H _{2} O \right)_{5} Cl \right] Cl _{2} \cdot H _{2} O$
  • $\left[ Co \left( H _{2} O \right)_{4} Cl _{2}\right] Cl \cdot 2 H _{2} O$
  • $\left[ Co \left( H _{2} O \right)_{3} Cl _{3}\right] \cdot 3 H _{2} O$
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The Correct Option is B

Solution and Explanation

Millimoles of $AgNO _{3}=\frac{1.2 \times 10^{22}}{6 \times 10^{23}} \times 1000=20$ Millimoles of $CoCl _{3} \cdot 6 H _{2} O =0.1 \times 100=10$ $\therefore$ Each mole of $CoCl _{3} \cdot 6 H _{2} O$ gives two chloride ions. $\therefore\left[ Co \left( H _{2} O \right)_{5} Cl \right] Cl _{2} \cdot H _{2} O$
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Concepts Used:

Werner’s Theory of Coordination Compounds

In 1893 Werner produced a theory to explain the structures, formation and nature of bonding in the coordination compounds. This theory is known as Werner’s theory of coordination compounds.

Postulates of Werner's Theory:

The important postulates as observed by Alfred Werner throughout his experiments are as follows:

  • The complex/ coordination compounds contain a central metal atom.
  • The metal atoms in a coordination compound generally show two types of valency: primary valency and secondary valency.
  • The primary valencies denote the oxidation state. They are ionizable and are satisfied by the negative ions.
  • Secondary valencies denote the coordination number. They are non-ionizable and are fixed for every metal atom. The secondary valency is generally satisfied by the neutral molecules or negative ions.
  • The metal atoms should satisfy both primary and secondary valencies.
  • The secondary valency of the atom basically shows the geometry/ polyhedra of the particular coordination compound.

Limitations of Werner’s Theory:

  • Though Werner explained some properties of the coordination compound,  he failed to explain the colour of the coordinate compound.
  • He could not explain the magnetic and optical properties of coordination compounds.
  • He could not answer the question, why does the coordination sphere have a definite geometry.