A blue crystalline salt dissolves in water to give a blue solution. Dilute HCl gives no precipitate, H2S in acid gives a black precipitate soluble in hot dilute HNO3, and excess ammonia gives a deep blue solution. The cation present and the spin-only magnetic moment of the deep blue complex ion are

JEE Advanced levelSingle correct optionDifficulty 3 of 3qc178

A blue crystalline salt dissolves in water to give a blue solution. Dilute HCl gives no precipitate, $H X_{2} S$ in acid gives a black precipitate soluble in hot dilute $HNO X_{3}$ , and excess ammonia gives a deep blue solution. The cation present and the spin-only magnetic moment of the deep blue complex ion are

No chloride precipitate rules out group I; black acid-insoluble sulphide and deep blue ammine identify

Cu X^{2 +}

. Cu(II) is

d^{9}

with one unpaired electron, so

μ = 1 (1 + 2) = 1.73 BM

. (Cu(I) would be

d^{10}

, 0 BM.)