Chemistry Electrochemistry PMDC 2020
PMDC Verified Question 43 of 73
The oxidation state of "S" in the \( S_2O_3^{-2} \) is:
A
+4
B
+6
C
-2
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 D: +2
Concept:

The average oxidation state of an element in a polyatomic ion is calculated by ensuring the sum of all oxidation states equals the net ionic charge.

Formula:

$$2x + 3(-2) = -2$$

Solution:

  • Let \( x \) be the average oxidation state of Sulfur (S) in the thiosulfate ion (\( S_2O_3^{-2} \)).


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


  • Set up the algebraic equation: \( 2x + 3(-2) = -2 \).


  • Solve for \( x \): \( 2x - 6 = -2 \implies 2x = +4 \implies x = +2 \).


Why other options are incorrect:

Sulfur has an oxidation state of \( +6 \) in sulfates (\( SO_4^{-2} \)) and \( +4 \) in sulfites (\( SO_3^{-2} \)). Thiosulfate specifically yields an average 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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