Hey there! Sign in to join this conversationNew here? Join for free
    • Thread Starter
    Offline

    3
    ReputationRep:
    This was actually a question from an old thread- just reposting it as I wanted some help with it.

    A solution is made from, 50cm^3 ,iron (ii) chloride, diluted to 500cm^3. the solution was then titrated against both 0.02M potassium manganate, and 0.01677M potassium dichromate (separately). (after adding an excess of dilute sulpuric acid). 25cm^3 solution was used each time, 25.50cm^3 more manganate was needed to be added each time than dichromate. calculate the concentration of iron (ii) chloride in the original solution. before dilution)
    the answer is 0.51M apparently

    Basically, this is my working so far:
    1) (Cr2O7)2- + 6Fe2+ + 14H+---->6Fe3+ + 2Cr3+ +7H2O
    2) MnO4- +5Fe2+ + 8H+----> 5Fe3+ + Mn2+ +4H2O
    Then, I set: Volume of (Cr2O72-)= x, and the Volume of (MnO4-)= x+25.5
    for reaction with Cr2O72-
    moles of Cr2O72-= 0.01677 X (x/1000)= (0.01677x)/1000
    moles of Fe2+ reacted in 25cm^3= 6(0.01677x)/1000= (0.10062x)/1000
    moles of Fe2+ in 500cm^3= 20(0.10062x)/1000= (2.0124x)/1000
    ^^^^This is then the number of moles in 50cm^3 also (before dilution)

    Then for the reaction with MnO4-:
    moles of MnO4-= 0.02 X (x+25.5)/1000
    moles of Fe2+ reacted in 25cm^3= moles above X5= 0.1(x+25.5)/1000
    moles of Fe2+ in 500cm^3= moles above X 20= 2(x+25.5)/1000
    ^^^^This is also the number of moles in 50cm^3 (before dilution)

    I then equated the two bolded values and came out with like x=4112.9....
    Where did I go wrong???
    • Thread Starter
    Offline

    3
    ReputationRep:
    charco could you possibly help out?
    Offline

    3
    ReputationRep:
    (Original post by thesmallman)
    This was actually a question from an old thread- just reposting it as I wanted some help with it.

    A solution is made from, 50cm^3 ,iron (ii) chloride, diluted to 500cm^3. the solution was then titrated against both 0.02M potassium manganate, and 0.01677M potassium dichromate (separately). (after adding an excess of dilute sulpuric acid). 25cm^3 solution was used each time, 25.50cm^3 more manganate was needed to be added each time than dichromate. calculate the concentration of iron (ii) chloride in the original solution. before dilution)
    the answer is 0.51M apparently

    Basically, this is my working so far:
    1) (Cr2O7)2- + 6Fe2+ + 14H+---->6Fe3+ + 2Cr3+ +7H2O
    2) MnO4- +5Fe2+ + 8H+----> 5Fe3+ + Mn2+ +4H2O
    Then, I set: Volume of (Cr2O72-)= x, and the Volume of (MnO4-)= x+25.5
    for reaction with Cr2O72-
    moles of Cr2O72-= 0.01677 X (x/1000)= (0.01677x)/1000
    moles of Fe2+ reacted in 25cm^3= 6(0.01677x)/1000= (0.10062x)/1000
    moles of Fe2+ in 500cm^3= 20(0.10062x)/1000= (2.0124x)/1000
    ^^^^This is then the number of moles in 50cm^3 also (before dilution)

    Then for the reaction with MnO4-:
    moles of MnO4-= 0.02 X (x+25.5)/1000
    moles of Fe2+ reacted in 25cm^3= moles above X5= 0.1(x+25.5)/1000
    moles of Fe2+ in 500cm^3= moles above X 20= 2(x+25.5)/1000
    ^^^^This is also the number of moles in 50cm^3 (before dilution)

    I then equated the two bolded values and came out with like x=4112.9....
    Where did I go wrong???
    Your chemical equations are correct. I think there is something wrong with the values. If you work forward from a supposed answer of 0.51 M, the titres for permanganate and chromate would be nearly the same.
    • Thread Starter
    Offline

    3
    ReputationRep:
    (Original post by TeachChemistry)
    Your chemical equations are correct. I think there is something wrong with the values. If you work forward from a supposed answer of 0.51 M, the titres for permanganate and chromate would be nearly the same.
    ohhhh okay cheers!
 
 
 
  • 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
    Would you rather give up salt or pepper?
  • 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

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