The conductivity of a solution of complex with formula CoCl₃(NH₃)₄ corresponds to 1 : 1 electrolyte, then the primary valency of central metal ion is __________.

Solution

Chemistry ma’am · 4 steps

Primary valency is the oxidation state of cobalt, and that does not change with how many chlorides ionise.

1. Read the conductivity. A 1 : 1 electrolyte gives one cation and one anion in solution. So only one Cl⁻ sits outside the coordination sphere, and the complex is [Co(NH₃)₄Cl₂]Cl.
2. Secondary valency. Inside the bracket, Co is bonded to 4 NH₃ and 2 Cl⁻. That makes the coordination number, or secondary valency, 6.
3. Primary valency. Primary valency is the oxidation state of the metal. Ammonia is neutral and each chloride is -1, so x + 4(0) + 3(-1) = 0 gives x = +3. All three chlorides balance this charge, including the two inside the sphere, which count toward both valencies.
4. So. Cobalt is in the +3 state, so its primary valency is 3.

The common slip: 1. Counting only the ionisable chloride gives 1, but primary valency is the oxidation state of Co, which is 3.

The idea to keep. Primary valency is the metal's oxidation state and secondary valency is its coordination number. Conductivity only tells you how many ions sit outside the bracket.

Answer from the NTA final answer key. Solution by PrepWiser.