Why moles of (S2O3 2-) is equal to mol of ( MnO4-) times 5

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Discussion Overview

The discussion revolves around the stoichiometric relationship between thiosulfate ions (S2O3 2-) and permanganate ions (MnO4-) in a redox titration context. Participants explore the underlying chemical reactions and the rationale for the mole ratio of S2O3 2- to MnO4- being 5:1, as derived from the balanced equations.

Discussion Character

  • Homework-related
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant presents the balanced chemical equations that establish the mole ratios, noting the 2:5 ratio of MnO4- to I2 and the 1:2 ratio of I2 to S2O3 2-.
  • Another participant suggests examining the half-reactions to understand the electron balance necessary for the reactions.
  • A different participant expresses uncertainty about the titration process, indicating that the goal may be to measure excess iodine rather than directly relating the moles of thiosulfate to permanganate.
  • One participant emphasizes the role of iodine as an intermediate in the reaction, suggesting that while the stoichiometric relationship can be derived, practical issues such as reaction speed and specificity complicate the titration process.
  • A later reply indicates that the explanation provided has clarified the participant's understanding of the topic.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the clarity of the titration process and the role of iodine, indicating that multiple views on the interpretation of the reactions and their implications remain.

Contextual Notes

Participants express uncertainty regarding the practical aspects of the titration and the implications of side reactions, which may affect the expected outcomes of the stoichiometric relationships.

Who May Find This Useful

This discussion may be useful for students studying redox titrations, particularly those interested in the stoichiometry of reactions involving thiosulfate and permanganate ions.

mimi88
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Homework Statement


The ratio of KMnO4 to I2 is given by
2 MnO4 - + 16 H+ + 10 I - ⇌ 2 Mn2+ + 5 I2 + 8 H2O
the ratio is 2:5

- The ratio of I2 to S2O3
2- is given by 1 I2 + 2 S2O32- ⇌ 2 I-+ S4O5 2-
the ratio is 1:2

therefore n(S2O3 2- ) = n(MnO4 - ) x 5

Homework Equations


Why the mole of (S2O3 2-) is equal to mol of ( MnO4-) times 5??
Where is the 5 come from?
What is the relationship between (S2O3 2- ) and ( MnO4-)?
Please help me, thanks.
 
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Look at each half-reaction! What is necessary to balance the electrons?
 
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My thought may have been off-track a little bit. Maybe you do have the reactions that are needed, but the titration is intended to find the excess iodine remaining from the first reaction.
 
symbolipoint said:
My thought may have been off-track a little bit. Maybe you do have the reactions that are needed, but the titration is intended to find the excess iodine remaining from the first reaction.

It is still about following electrons - we use iodine as an intermediate to make the chemistry efficient, but in the end thiosulfate is oxidized by permanganate. This can be written as a single, hypothetical reaction and then the ratio between thiosulfate and permanganate is obvious. The problem is, such reaction doesn't have properties we expect from reactions used in titration - it is either not fast enough, nor specific enough (side reactions are often a problem in such cases).
 
Thank guys, now it makes more sense to me:)
 

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