utv
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I would find the number of moles of k2cr2o7 by doing 1.19/Mr

divide the moles by 10 as we're using only 25cm3

as the ratio is 1:3

3 moles of iodine is formed


times by 2 to find the moles of s2o3 then divide by the volume to.find the conc

my teacher said you don't do that but you divide by 3 instead then times by 2. can anyone tell me why
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Kian Stevens
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Your methodology is correct up until your mistake in the stoichiometric ratios

In the first equation, yes the ratio of dichromate to iodine is 1:3
However, in the second equation the ratio of thiosulfate to iodine is 2:1

So to 'convert' from 3 equivalents of iodine to just 1 equivalent you divide by three; and then, using the stoichiometry of the second equation, you then multiply by two to find the moles of dichromate

You can also see it like this: the moles of iodine formed in the first equation are such that the ratio of thiosulfate to iodine is essentially 2:3 anyway
Thus, using normal stoichiometric rules, to find the moles of thiosulfate you have to divide by three and multiply by two
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utv
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thank you. would you suggest working backwards with these questions. as would it make it easier. I thought that 3 mol is formed and from that 3 mol the other reaction occurs in a 1:2 ratio hence 6mol of s2o3. or do you take it literate with both equation and try to equate them into one.
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Kian Stevens
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(Original post by utv)
thank you. would you suggest working backwards with these questions. as would it make it easier. I thought that 3 mol is formed and from that 3 mol the other reaction occurs in a 1:2 ratio hence 6mol of s2o3. or do you take it literate with both equation and try to equate them into one.
It depends what the objective of the question is: if you're determining the concentration of thiosulfate (like here) then no, but if you're using the concentration of thiosulfate for something then yes

As for your second question: it doesn't necessarily work like that, and if you consider what I said previously it makes sense as to why
You've kind of just got to imagine that both reactions occur in concert so whilst they're not combined in the question, you have to imagine that they are in order for it to make sense why the stoichiometry works in this particular way

Furthermore, imagine that you were reading the question backwards: if you had some moles of thiosulfate and you had to determine the moles of iodine in the first equation, you'd obviously divide the moles of thiosulfate by two to get the moles of iodine in the second equation, and then multiply by three to get the moles of iodine in the first equation; it's the exact opposite here
Last edited by Kian Stevens; 8 months ago
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utv
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thank you😊
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