A compound of cobalt(III) chloride with ammonia, CoCl3.xNH3, is one of the complexes studied by Werner; cobalt in it has a secondary valence of six. When 0.020 mol of the compound is treated with an excess of silver nitrate solution in cold, 5.74 g of AgCl is precipitated. What is the value of x? (Molar mass of AgCl = 143.5 g mol−1)
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → 5
Only the chloride ions outside the square brackets are precipitated as AgCl. The chloride ions that are not precipitated stay bound to cobalt. Together with the ammonia molecules they make up the six groups bound to cobalt.
\( \displaystyle n(\ce{AgCl}) = \frac{5.74\ \mathrm{g}}{143.5\ \mathrm{g\,mol^{-1}}} = 0.040\ \mathrm{mol} \)
\( \displaystyle \text{mol of AgCl per mol of compound} = \frac{0.040}{0.020} = 2 \)
\( \displaystyle \text{Cl bound to Co} = 3 - 2 = 1 \)
\( \displaystyle x = 6 - 1 = 5 \)
The compound is CoCl3.5NH3, formulated as \( \ce{[CoCl(NH3)5]^2+ 2Cl-} \).
NCERT (2026-27 edition):
5.1 Werner’s Theory of Coordination Compounds – “…In a series of compounds of cobalt(III) chloride with ammonia, it was found that some of the chloride ions could be precipitated as AgCl on adding excess silver nitrate solution in cold but some remained in solution.”
“1 mol CoCl3.6NH3 (Yellow) gave 3 mol AgCl
1 mol CoCl3.5NH3 (Purple) gave 2 mol AgCl
1 mol CoCl3.4NH3 (Green) gave 1 mol AgCl
1 mol CoCl3.4NH3 (Violet) gave 1 mol AgCl”
“These observations, together with the results of conductivity measurements in solution can be explained if (i) six groups in all, either chloride ions or ammonia molecules or both, remain bonded to the cobalt ion during the reaction and (ii) the compounds are formulated as shown in Table 5.1, where the atoms within the square brackets form a single entity which does not dissociate under the reaction conditions.”
Table 5.1: Formulation of Cobalt(III) Chloride-Ammonia Complexes