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# redox titration watch

1. So this is the redox titration question:

1. )A student weighed out 5.56 g of FeSO4·7H2O, dissolved it in dilute sulfuric acid and made up the solution to 250 cm3 in a volumetric flask using distilled water. She then titrated 25 cm3 samples of this solution against potassium manganate(VII) solution. The iron(II) sulfate solution required 20.0 cm3 for complete reaction.
(a) Calculate the concentration of the Fe2+ ion in the iron(II) sulfate solution.
(b) Calculate the concentration of the potassium manganate solution.

in part a it asks for the concentration of fe2+ ion in the iron sulfate, why in the book have they done m/Mr, isn't that working out moles?

Im guessing in part b once you work out moles of the salt it is the same as the moles of fe2+ and then you scale this using your ratio (5:1) to the amount of manganate and then use the n=CxV equation using the 20 cm3 as the volume to get the concentration.
2. (Original post by Xenilbohl)
So this is the redox titration question:

1. )A student weighed out 5.56 g of FeSO4·7H2O, dissolved it in dilute sulfuric acid and made up the solution to 250 cm3 in a volumetric flask using distilled water. She then titrated 25 cm3 samples of this solution against potassium manganate(VII) solution. The iron(II) sulfate solution required 20.0 cm3 for complete reaction.
(a) Calculate the concentration of the Fe2+ ion in the iron(II) sulfate solution.
(b) Calculate the concentration of the potassium manganate solution.

in part a it asks for the concentration of fe2+ ion in the iron sulfate, why in the book have they done m/Mr, isn't that working out moles?

Im guessing in part b once you work out moles of the salt it is the same as the moles of fe2+ and then you scale this using your ratio (5:1) to the amount of manganate and then use the n=CxV equation using the 20 cm3 as the volume to get the concentration.
Concentration is measured in moles per litre, so you must calculate the moles and then divide by the number of litres.
3. (Original post by charco)
Concentration is measured in moles per litre, so you must calculate the moles and then divide by the number of litres.
I get that it is moles per litre which is the same as moles per dm3, but what I dont understand is why in the book they just work out the number of moles? the salt is dissolved in 250cm3, wouldnt i therefore need to divide the number of moles by 0.25?
4. (Original post by Xenilbohl)
I get that it is moles per litre which is the same as moles per dm3, but what I dont understand is why in the book they just work out the number of moles? the salt is dissolved in 250cm3, wouldnt i therefore need to divide the number of moles by 0.25?
Yes, it looks like the book has answered the question, "How many moles of Fe2+ are in 5.56 g of the salt?"

.. it's not unusual to get mistakes in textbooks.
5. (Original post by charco)
Yes, it looks like the book has answered the question, "How many moles of Fe2+ are in 5.56 g of the salt?"

.. it's not unusual to get mistakes in textbooks.
PHEW, thanks a lot!! I was getting worried because my exams not to far off...

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