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Homework Help: Mole calculation

  1. Sep 24, 2008 #1
    1.0 g of Iron metal reacts with 2.0 g of chlorine gas to make iron (III) chloride, FeCl3.

    (a) Write a balanced equation for this reaction.
    (b) Which of the reactants is present in excess ?
    (c) What mass of this reactant will be left over at the end of the reaction ? (7 marks)

    2. Relevant equations

    OK with equation : 2Fe + 3Cl2 = 2FeCl3

    3. The attempt at a solution

    Went for moles of Fe as 2/112 and moles of Cl2 as 2/213 then didn't know what to do with numbers
  2. jcsd
  3. Sep 24, 2008 #2


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    Welcome to PF!

    Hi sues! Welcome to PF! :smile:

    Hint: how many moles in 1g of iron and in 1g of chlorine? :smile:
  4. Sep 24, 2008 #3
    Thanks Tiny Tim.
    So if I did the following am I on the right track?
    Moles of Fe = 1/112 and moles of Cl2 = 2/213 (taking into account stoiciometry).
    Therefore Cl2 in excess.

    Mass of Cl2 left after reaction:

    9.39x10-3 - 8.92x10-3 = 4.7x10-4

    Mass = 4.7x10-4 x 71 = 0.0334g

    Any good?
  5. Sep 24, 2008 #4


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    Where are you getting these numbers?

    - Convert one of the reagents into moles

    - Next find out the mole equivalence of the other reagent e.g. if you used Chlorine gas then note that there is two Iron III for every three Chlorine gases , use this ratio to find the moles of Chlorine then convert this value into grams

    - Compare this value with the one given to you to understand it is going to suffice then you know the limiting reagent.

    Show your work then we can move on to the second part.
  6. Sep 24, 2008 #5
    Ok here goes!
    moles of Cl2= 2/71 = 0.028
    moles of Fe = 2/3 x 0.028 = 0.0187
    mass of Fe = 0.0187 x 57 = 1.064g

    or other way

    moles of Fe = 0.018
    moles of l2 = 0.027
    mass of Cl2 = 1.917g therefore Cl2 in excess.

    mass of reactant in excess left over = 2.0 - 1.917 = 0.083g

  7. Sep 24, 2008 #6


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    Staff: Mentor

    OK, although your numbers are slightly off, you have either used some strange molar masses or rounded intermediate results.

    Red means in excess.

    Attached Files:

  8. Sep 24, 2008 #7


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    Great! Just be certain to round off those numbers correctly. BTW I haven't checked on whether the numbers for the second portion are correct since you did not display your work here.
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