Expressing Concentration Of Solutions

The formula of the complex is CoCl₃(NH₃)₄. In coordination chemistry, the term "primary valency" refers to the oxidation state of the central metal ion. To determine this, we must consider electrolyte behavior and overall charge.

Given that the solution behaves like a 1:1 electrolyte, it indicates the complex produces two ions when dissolved: the complex ion and one chloride ion. For a 1:1 electrolyte:

CoCl₃(NH₃)₄ ⇌ [Co(NH₃)₄Cl]²⁺ + Cl^-

The complex ion [Co(NH₃)₄Cl]²⁺ suggests that other 2 chlorides, contributing to the primary valency, are bonded outside the coordination sphere. The complex ion has a charge of +2. Therefore, the oxidation state or primary valency of Co must account for the charge of the complex ion plus the charge of outer-sphere chlorides:

Charge balance: x - 3 (+1) = +2, where x is the oxidation state of Co.

Solving: x - 3 = 2 implies x = 3.

The primary valency of the central metal ion is +3, verified against the provided range (3,3), which confirms the result.

Aromatic aldehydes do not give Fehling’s test. Both nitrogen and sulfur must be present to obtain the blood red colour. Sodium nitroprusside gives blood red colour with nitrogen and sulfur. Therefore, the compound in option (4) is the correct answer.