You are Here: Home

# Redox Titration - Fermentation of Sucrose and Solution R watch

1. I have an issue with this question, in particular calculating the concentration of ethanol in sample Q. I've attached parts of question (the original paper here; question 21a i-v) and the mark scheme to the question is found here and the examiner report here

I understand pretty much everything but my issue is part v.
From the previous parts of the question, I calculate the mole ratio of the reaction between Fe(ii) and dichromate ions, then use the mole ratio to calculate the moles of dichromate reacted with Fe(II) and then in the third part, the moles of dichromate reacted with ethanol. Then calculate the mole ratio of the reaction between ethanol and dichromate.
But then in last part of the question, I used the mole ration of the reaction between ethanol and dichromate (3:2 respectively) to calculate the moles of ethanol in 25cm3 and then in 250cm3 (by x10) however there's a problem now....
I can't calculate the concentration of ethanol for some reason in solution Q because I don't know the volume of solution Q. According to the mark scheme, the volume is 40 for some reason but I'm confused as to why it is 40 though. The stem doesn't give that number. The provided answer in the examiner report also uses 40 by a ratio but I'm confused where the student gets the ratio in the report though.
Why on earth do I multiply by 40?
Attached Images

2. (Original post by Feraligatr)

I have an issue with this question, in particular calculating the concentration of ethanol in sample Q. I've attached parts of question (the original paper here; question 21a i-v) and the mark scheme to the question is found here and the examiner report here

I understand pretty much everything but my issue is part v.
From the previous parts of the question, I calculate the mole ratio of the reaction between Fe(ii) and dichromate ions, then use the mole ratio to calculate the moles of dichromate reacted with Fe(II) and then in the third part, the moles of dichromate reacted with ethanol. Then calculate the mole ratio of the reaction between ethanol and dichromate.
But then in last part of the question, I used the mole ration of the reaction between ethanol and dichromate (3:2 respectively) to calculate the moles of ethanol in 25cm3 and then in 250cm3 (by x10) however there's a problem now....
I can't calculate the concentration of ethanol for some reason in solution Q because I don't know the volume of solution Q. According to the mark scheme, the volume is 40 for some reason but I'm confused as to why it is 40 though. The stem doesn't give that number. The provided answer in the examiner report also uses 40 by a ratio but I'm confused where the student gets the ratio in the report though.
Why on earth do I multiply by 40?
Your calculation determines the moles of ethanol in 25ml of solution. To get moles per litre you have to multiply by 40 ...
3. Just as a matter of interest, would this practical actually work?

If so, it looks worth pursuing as an interesting experiment.

However, the reduced dichromate is dark green (chromium(III)) and you are trying to titrate the orange unreacted dichromate (which will also turn green). I can't see how the end-point can possibly be seen. Anyone out there tried it?

@pigster
@ajsullivan
@teachchemistry
4. (Original post by charco)
Just as a matter of interest, would this practical actually work?

If so, it looks worth pursuing as an interesting experiment.

However, the reduced dichromate is dark green (chromium(III)) and you are trying to titrate the orange unreacted dichromate (which will also turn green). I can't see how the end-point can possibly be seen. Anyone out there tried it?

@pigster
@ajsullivan
@teachchemistry
THANK YOU SO MUCH. DM^-3 = LITRE

With regards to the point you're making, I don't think it'd be feasible considering the solution is already green enough
Another question in the same paper asks
"This method of determining ethanol concentration does not give very reliableresults, although the titration is very accurate. Suggest one reason why this might be the case, explaining how the measuredconcentration would differ from the actual concentration of the ethanol."

The mark scheme mentions that it is difficult to know when the reaction is complete.
5. (Original post by charco)
Just as a matter of interest, would this practical actually work?

If so, it looks worth pursuing as an interesting experiment.

However, the reduced dichromate is dark green (chromium(III)) and you are trying to titrate the orange unreacted dichromate (which will also turn green). I can't see how the end-point can possibly be seen. Anyone out there tried it?

@pigster
@ajsullivan
@teachchemistry
You won't get a nice end point with the colour change from Cr2O72- to Cr3+, but there is a selection of indicators that you can used: diphenylamine, diphenylbenzidine or diphenylamine sulfonate.
6. (Original post by Pigster)
You won't get a nice end point with the colour change from Cr2O72- to Cr3+, but there is a selection of indicators that you can used: diphenylamine, diphenylbenzidine or diphenylamine sulfonate.
... and how would these indicators not be masked by the green colour of the solution.

Have you carried out the experiment?
7. Many years ago I tried it without an indicator and it was hopeless.

I haven't tried it with an indicator myself, my colleagues have done a similar prac:
Fe2+ in solution with diphenylamine sulfonate indicator and H3PO4 (to complex the Fe3+ and prevent it oxidising the indicator). Cr2O72- was added and a violet colour change seen on completion.

### Related university courses

TSR Support Team

We have a brilliant team of more than 60 Support Team members looking after discussions on The Student Room, helping to make it a fun, safe and useful place to hang out.

This forum is supported by:
Updated: February 4, 2016
Today on TSR

### Exam Jam 2018

Join thousands of students this half term

### Solo - A Star Wars Story

Poll

The Student Room, Get Revising and Marked by Teachers are trading names of The Student Room Group Ltd.

Register Number: 04666380 (England and Wales), VAT No. 806 8067 22 Registered Office: International House, Queens Road, Brighton, BN1 3XE