Hey there! Sign in to join this conversationNew here? Join for free

Redox Titration - Fermentation of Sucrose and Solution R Watch

Announcements
    • Welcome Squad
    • Thread Starter
    Offline

    13
    ReputationRep:
    :cookie:

    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? :indiff:
    Attached Images
        
    • Community Assistant
    • Study Helper
    Offline

    14
    ReputationRep:
    (Original post by Feraligatr)
    :cookie:

    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? :indiff:
    Your calculation determines the moles of ethanol in 25ml of solution. To get moles per litre you have to multiply by 40 ...
    • Community Assistant
    • Study Helper
    Offline

    14
    ReputationRep:
    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
    • Welcome Squad
    • Thread Starter
    Offline

    13
    ReputationRep:
    (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 :indiff:

    With regards to the point you're making, I don't think it'd be feasible considering the solution is already green enough :indiff:
    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.
    Offline

    16
    ReputationRep:
    (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.
    • Community Assistant
    • Study Helper
    Offline

    14
    ReputationRep:
    (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?
    Offline

    16
    ReputationRep:
    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.
 
 
 
  • See more of what you like on The Student Room

    You can personalise what you see on TSR. Tell us a little about yourself to get started.

  • Poll
    Should Spain allow Catalonia to declare independence?
  • See more of what you like on The Student Room

    You can personalise what you see on TSR. Tell us a little about yourself to get started.

  • 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

    Quick reply
    Reputation gems: You get these gems as you gain rep from other members for making good contributions and giving helpful advice.