JEE Advanced 2021 · Paper 1 · Chemistry Q4

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ChemistrySingle correct+3 / -1Question 23 of 57

The calculated spin only magnetic moments of $\ce{[Cr(NH3)6]^{3+}}$ and $\ce{[CuF6]^{3-}}$ in BM, respectively, are

(Atomic numbers of Cr and Cu are 24 and 29, respectively)

3.87 and 2.84

4.90 and 1.73

3.87 and 1.73

4.90 and 2.84

Answer and worked solution

Official answer: A

$\ce{[Cr(NH3)6]^{3+}}$ . $\ce{Cr^{3+}}$ is $d^3$ ( $t_{2g}^3$ ), with 3 unpaired electrons:
$\mu=\sqrt{3\cdot5}=3.87\text{ BM}.$

$\ce{[CuF6]^{3-}}$ . $\ce{Cu^{3+}}$ is $d^8$ ; in an octahedral field ( $t_{2g}^6e_g^2$ ) it has 2 unpaired electrons:
$\mu=\sqrt{2\cdot4}=2.83\approx2.84\text{ BM}.$
(A)

Topics:Magnetic properties and colour Crystal field theory

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