Chemistry S & P Block Elements SZABMU 2023
PMDC Verified Question 33 of 104
The oxidation number of oxygen in \( \text{Na}_2\text{O}_2 \) is:
A
-1
B
+2
C
-2
D
+1
Tap any option to test your recall and reveal the step-by-step Propolis autopsy.

Propolis Cognitive Error Autopsy

Official Correct Choice:
Option A: -1
Concept:

While Oxygen normally has an oxidation state of -2, it exhibits different states when bonded to itself in peroxides or superoxides.

Solution:

  • The compound \( \text{Na}_2\text{O}_2 \) is Sodium peroxide.
  • Alkali metals (like Na) always have an oxidation state of +1 in their compounds.
  • Let the oxidation number of Oxygen be \( x \). Set up the algebraic equation for the neutral molecule:

    $$ 2(\text{Na}) + 2(\text{O}) = 0 $$
    $$ 2(+1) + 2(x) = 0 $$
    $$ 2 + 2x = 0 $$
    $$ 2x = -2 $$
    $$ x = -1 $$

  • Therefore, the oxidation number of each oxygen atom in a peroxide is -1.


Why other options are incorrect:

  • -2: This is for normal oxides (e.g., \( \text{Na}_2\text{O} \)).
  • +1 / +2: Oxygen only exhibits positive oxidation states when bonded to the more electronegative Fluorine (e.g., \( \text{OF}_2 \)).

Quality & Fidelity Assurance: Every question on BeambePrep is rigorously curated against the official PMDC syllabus with zero filler, zero out-of-syllabus content, and zero typos. When an authentic past paper originally contained a historical mistake or ambiguity from the examining board (such as UHS or NUMS), BeambePrep faithfully reflects the original paper while detailing the nuance and scientific consensus in the autopsy above.

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