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

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SUMMARY

The mole ratio of thiosulfate ions (S2O3 2-) to permanganate ions (MnO4-) is established as 5:1 based on the stoichiometry of the redox reactions involved. The reaction between potassium permanganate (KMnO4) and iodine (I2) produces a 2:5 ratio, while the subsequent reaction between iodine and thiosulfate yields a 1:2 ratio. Consequently, the relationship can be expressed as n(S2O3 2-) = n(MnO4-) x 5, clarifying the stoichiometric coefficients derived from the balanced half-reactions. Understanding these relationships is crucial for accurate titration calculations in redox chemistry.

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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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