JEE Challenger
More from Chemical Bonding and Molecular Structure
ChemMedAdv 2020 • Paper 2

Magnetic Response of Substances in External Magnetic Field Balance

In a given experimental setup, mm grams of a substance X\mathbf{X} (which can be a gas, liquid, or solid) is loaded onto a balance container, as depicted in figure I\mathbf{I}. Upon applying a magnetic field, the pan holding X\mathbf{X} either shifts upwards (figure II\mathbf{II}) or downwards (figure III\mathbf{III}), determined by the magnetic nature of compound X\mathbf{X}. Select the correct statement(s).

Question Diagram 1

Options

A

If X\mathbf{X} is H2O(l)\text{H}_2\text{O}(l), deflection of the pan is upwards.

Correct
B

If X\mathbf{X} is K4[Fe(CN)6](s)\text{K}_4[\text{Fe}(\text{CN})_6](s), deflection of the pan is upwards.

Correct
C

If X\mathbf{X} is O2(g)\text{O}_2(g), deflection of the pan is downwards.

Correct
D

If X\mathbf{X} is C6H6(l)\text{C}_6\text{H}_6(l), deflection of the pan is downwards.

Topics & Concepts

Chemical Bonding and Molecular StructureMolecular Orbital TheoryCoordination Compounds

Step-by-Step Solution

To determine the behavior of substance X\mathbf{X} in the presence of an external magnetic field using Gouy's balance method:

  1. Diamagnetic Substances: Substances with no unpaired electrons are weakly repelled by a magnetic field. Since the pole pieces of the magnet are located beneath/around the sample pan X\mathbf{X}, a repulsive magnetic force acts upwards, pushing the sample pan away from the magnetic field. Hence, for a diamagnetic substance, an upward deflection (Figure II\mathbf{II}) is observed.

  2. Paramagnetic Substances: Substances with unpaired electrons are attracted into a magnetic field. The attractive magnetic force pulls the sample pan downwards into the region of maximum magnetic field strength. Hence, for a paramagnetic substance, a downward deflection (Figure III\mathbf{III}) is observed.

Analysis of the Options:

  • Option (A): H2O(l)\text{H}_2\text{O}(l)

    • Water contains only paired electrons in all its chemical bonds and lone pairs.
    • Therefore, H2O(l)\text{H}_2\text{O}(l) is diamagnetic.
    • Being diamagnetic, it experiences a repulsive force and causes an upward deflection of the pan.
    • Statement (A) is CORRECT.

  • Option (B): K4[Fe(CN)6](s)\text{K}_4[\text{Fe}(\text{CN})_6](s)

    • In the complex ion [Fe(CN)6]4−[\text{Fe}(\text{CN})_6]^{4-}, iron is in the +2+2 oxidation state (Fe2+\text{Fe}^{2+}, d6d^6 configuration).
    • CN−\text{CN}^- is a strong field ligand, causing maximum pairing of dd-electrons: Electronic configuration: t2g6eg0\text{Electronic configuration: } t_{2g}^6 e_g^0
    • Since the number of unpaired electrons n=0n = 0, the compound is diamagnetic.
    • Being diamagnetic, it experiences an upward deflection of the pan.
    • Statement (B) is CORRECT.

  • Option (C): O2(g)\text{O}_2(g)

    • According to Molecular Orbital Theory (MOT), the molecular orbital electronic configuration of O2\text{O}_2 is: σ1s2 σ1s∗2 σ2s2 σ2s∗2 σ2pz2 (π2px2=π2py2) (π2px∗1=π2py∗1)\sigma_{1s}^2\, \sigma_{1s}^{*2}\, \sigma_{2s}^2\, \sigma_{2s}^{*2}\, \sigma_{2p_z}^2\, (\pi_{2p_x}^2 = \pi_{2p_y}^2)\, (\pi_{2p_x}^{*1} = \pi_{2p_y}^{*1})
    • It contains 22 unpaired electrons in the degenerate π∗\pi^* antibonding molecular orbitals (n=2n = 2).
    • Therefore, O2(g)\text{O}_2(g) is paramagnetic.
    • Being paramagnetic, it is pulled into the magnetic field, causing a downward deflection of the pan.
    • Statement (C) is CORRECT.

  • Option (D): C6H6(l)\text{C}_6\text{H}_6(l)

    • Benzene (C6H6\text{C}_6\text{H}_6) has a closed-shell aromatic electronic structure with all electrons fully paired.
    • Therefore, C6H6(l)\text{C}_6\text{H}_6(l) is diamagnetic.
    • Being diamagnetic, it should cause an upward deflection, not a downward deflection.
    • Statement (D) is INCORRECT.

Conclusion:

The correct statements are A, B, and C.

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