Q13 / 150 · MHT CET 2025 (21 Apr Shift 1) · Chemistry

Which is NOT correct regarding stock notation?

  • A
    $\mathrm{Hg}_2$ (I) $\mathrm{Cl}_2$ is reduced form of Hg (II) $\mathrm{Cl}_2$
  • B
    Hg (II) $\mathrm{Cl}_2$ is oxidised form of $\mathrm{Hg}_2$ (I) $\mathrm{Cl}_2$
  • C
    It helps to identify whether species is present in oxidised form or reduced form
  • Correct answer
    D
    Aurous chloride is written as Au (III) $\mathrm{Cl}_3$

Solution

The stock notation uses Roman numerals to indicate the oxidation state of the metal in a compound.

In $\mathrm{Hg_2Cl_2}$, mercury has oxidation state +1 (since $2 \times (+1) + 2 \times (-1) = 0$), denoted $\mathrm{Hg(I)}$. In $\mathrm{HgCl_2}$, mercury has oxidation state +2 (since $+2 + 2 \times (-1) = 0$), denoted $\mathrm{Hg(II)}$. Since +1 is a lower oxidation state than +2, $\mathrm{Hg_2Cl_2}$ is the reduced form of $\mathrm{HgCl_2}$, confirming statement A is correct.

If $\mathrm{Hg_2Cl_2}$ is reduced, then $\mathrm{HgCl_2}$ is its oxidized form, validating statement B.

Stock notation indicates the oxidation state, allowing comparison of redox states, so statement C is correct.

Aurous chloride corresponds to gold(+1), $\mathrm{AuCl}$, not $\mathrm{Au(III)Cl_3}$ (which is auric chloride). Statement D is incorrect.

The incorrect statement is D.

Final answer: $\boxed{D}$

Hence, the correct answer is option: 'D'.

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