MCQ
Given below are two statements, one is labelled as Assertion $A$ and the other is labelled as Reason $R$.

Assertion $A:$ The spin only magnetic moment value for $\left[ Fe ( CN )_6\right]^{3-}$ is $1.74 BM$, whereas for $\left[ Fe \left( H _2 O \right)_6\right]^{3+}$ is $5.92\,BM$.

Reason $R :$ In both complexes, $Fe$ is present in +3 oxidation state.

In the light of the above statements, choose the correct answer from the options given below:

  • Both $A$ and $R$ are true but $R$ is NOT the correct explanation of $A$
  • B
    $A$ is false but $R$ is true
  • C
    $A$ is true but $R$ is false
  • D
    Both $A$ and $R$ are true and $R$ is the correct explanation of $A$

Answer

Correct option: A.
Both $A$ and $R$ are true but $R$ is NOT the correct explanation of $A$
a
Unpaired electron $=1$

$\mu=\sqrt{ n ( n +2)}=\sqrt{1 \times 3}=1.74 \text { B.M. }$

$\left[ Fe \left( H _2 O \right)_6\right]^{3+}$ No pairing because $H _2 O$ is WFL Number of unpaired electrons $=5, \mu=5.92\,BM$ Assertion is true, Reason is true but not correct explanation.

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