Exams › JEE Main › Chemistry

When a pink aqueous solution of [Co(H2O)6]2+ is treated with an excess of concentrated HCl, a coloured complex is formed. What is the correct combination of the colour and spin-only magnetic moment of this product?

1. Blue colour and magnetic moment 3.87 BM
2. Blue colour and magnetic moment 1.73 BM
3. Pink colour and magnetic moment 3.87 BM
4. Pink colour and magnetic moment 5.92 BM

Correct answer: Blue colour and magnetic moment 3.87 BM

Solution

Adding excess conc. HCl converts the octahedral hexaaqua ion [Co(H2O)6]2+ (pink) into the tetrahedral tetrachlorocobaltate(II) ion [CoCl4]2- (blue). Co2+ is a d7 ion; in the weak tetrahedral field of Cl- it remains high-spin with 3 unpaired electrons, giving a spin-only moment mu = sqrt(3*(3+2)) = sqrt(15) = 3.87 BM. The colour change pink -> blue is the classic test used in the cobalt chloride humidity indicator.

Related JEE Main Chemistry questions

• When sodium hydroxide is introduced into an aqueous Zn2+ solution, a white precipitate forms, and this precipitate dissolves when more NaOH is added. In such a solution, zinc is present in the:
• When excess ammonia is added to a copper sulphate solution, a deep blue colour appears because of the formation of which species?
• In the nitroprusside ion, iron and NO are present as Fe2+ and NO+ instead of Fe3+ and NO. These two formulations can be distinguished by
• In the brown ring test for nitrite and nitrate ions, the characteristic brown ring is produced because a complex ion is formed with which formula?
• What is the spin-only magnetic moment, expressed in Bohr magnetons (μB), for Ni2+ in aqueous solution? (Atomic number of Ni = 28)
• Silver chloride dissolves in aqueous ammonium hydroxide because it leads to the formation of which species?

⚔️ Practice JEE Main Chemistry free + battle 1v1 →