Werner's Theory

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When excess silver nitrate is added to one mole of the complex CoCl3.6NH3 in aqueous solution, how many moles of AgCl are precipitated?

RankGuru · Accepted Answer

Werner distinguished between primary valence, satisfied by ionisable anions, and secondary valence, satisfied by directly bonded groups inside the coordination sphere. In CoCl3.6NH3, all six ammonia molecules occupy the secondary valences around cobalt, so the formula is written as [Co(NH3)6]Cl3. The three chloride ions lie outside the coordination sphere and are freely ionisable, giving the complex cation [Co(NH3)6]^{3+} plus 3 Cl^- in solution. Adding excess silver nitrate precipitates every free chloride as AgCl, so 3 moles of AgCl form. The option of 1 mole corresponds to [Co(NH3)4Cl2]Cl, and 2 moles corresponds to [Co(NH3)5Cl]Cl2, both of which have chloride inside the sphere and are therefore wrong here. Zero precipitate is impossible because not all chlorides are coordinated. This experiment is the classic NCERT illustration that conductivity and precipitation reveal how many ions a complex releases. Plausibility check: three ionisable chlorides match the 1:3 electrolyte behaviour expected for a tripositive complex cation, so the answer is internally consistent.

Werner's Theory

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