Chemistry Electrochemistry NUMS 2019
PMDC Verified Question 46 of 73
The oxidation state of carbon in \( C_2O_4^{-2} \) is:
A
+4
B
-4
C
+3
D
+2
Tap any option to test your recall and reveal the step-by-step Propolis autopsy.

Propolis Cognitive Error Autopsy

Official Correct Choice:
Option C: +3
Concept:

In polyatomic ions, the algebraic sum of the oxidation numbers of all atoms equates to the overall charge of the ion.

Formula:

$$2x + 4(-2) = -2$$

Solution:

  • Let \( x \) be the oxidation state of a single Carbon (C) atom.


  • The oxidation state of Oxygen (O) is \( -2 \).


  • Set up the equation for the oxalate ion (\( C_2O_4^{-2} \)): \( 2x + 4(-2) = -2 \).


  • Solve for \( x \): \( 2x - 8 = -2 \implies 2x = +6 \implies x = +3 \).


Why other options are incorrect:

Carbon has an oxidation state of \( +4 \) in carbon dioxide (\( CO_2 \)) and carbonates, not in oxalates. \( -4 \) is for methane (\( CH_4 \)).

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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