Assertion (A): [Co(NH_3)_6]³+ is diamagnetic. Reason (R): NH_3 is a strong field ligand and causes pairing of electrons. Select the most appropriate answer from the options given below: (A) Both A and R are true, and R is the correct explanation of A. (B) Both A and R are true, and R is not the correct explanation of A. (C) A is true but R is false. (D) A is false but R is true

ChemistryCBSE Class 12 · Sample paper, session 2026-27 · 1 mark · Assertion-reason

CBSE Class 12 Chemistry sample paper 2026-27: question 16

From the official CBSE sample question paper for 2026-27, section A, question 16, for 1 mark. Try it first; the hints and our step-by-step solution are below.

Q16 · 1 mark · Assertion-reasonCompetency-basedCoordination Compounds

Assertion (A): $[C o (N H_{3})_{6}]^{3 +}$ is diamagnetic.
Reason (R): $N H_{3}$ is a strong field ligand and causes pairing of electrons.

Select the most appropriate answer from the options given below:

(A)Both A and R are true, and R is the correct explanation of A.
(B)Both A and R are true, and R is not the correct explanation of A.
(C)A is true but R is false.
(D)A is false but R is true

Source: official sample paper from cbseacademic.nic.in (PDF). See it in the full Sample paper 2026-27.

Hints

Open one hint at a time and try again before the next.

Hint 1

Write the d-electron count of Co³⁺.

Hint 2

Think about how a strong field ligand affects electron pairing.

Step-by-step solution

Each step shows the marks it earns, as an examiner would award them. Checked by an independent re-solve from the question paper.

$C o^{3 +}$ has a $3 d^{6}$ configuration. NH₃ is a strong field ligand, so all six electrons pair in the $t_{2 g}$ set, giving $t_{2 g}^{6} e_{g}^{0}$ with $d^{2} s p^{3}$ hybridisation.
With no unpaired electrons the complex is diamagnetic. The pairing caused by the strong field NH₃ is the reason, so R correctly explains A.
1 mark

Answer: Option (A)

Where the marks are

States $t_{2 g}^{6}$ with all electrons paired by strong field NH₃, so R explains A (1 mark)

Revise the chapter

Chapter: Coordination Compounds (notes, formulas, where marks are lost and every past question)
Topic: Crystal Field Theory

Practise this chapterBoard-style questions from Coordination Compounds, one at a time, marked against the answer key and marking points.Back to the full paperSample paper 2026-27: every question, by section.

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CBSE Class 12 Chemistry sample paper 2026-27: question 16

Hints

Step-by-step solution

Revise the chapter

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