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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.

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