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Moles and Percent Composition

  1. Jan 16, 2016 #1
    1. The problem statement, all variables and given/known data
    A 16.00g sample of a mixture of KClO3 and KCl is heated until all the oxgen has been removed from the sample. The product entirely KCl (no O), has a total mass of 9.00g. What is the percentage of KClO3 present in the original mixture?

    16.00g mixture
    9.00g mixture without oxygen
    3.00g oxygen without mixture

    2. Relevant equations
    Percent Composition (percent = mass of element/mass of compound)
    Various mole equations (eg. moles to mass)

    3. The attempt at a solution
    I received this question on a test awhile back and, while I have received the answer and the steps to reach it, I cannot understand why it is that it is done that way. From what I can see, we do not need to worry about percent composition of the mixture until the end. At the moment, we do not have KCl by itself, as O3 is bonded to it.

    I think I need to find the mass of KCl in the original mixture, and so to do that, I chose to calculate the moles in KCl and O.

    9.00gKCl/(75g/mol) = 0.12molKCl
    3.00gO/(16g/mol)= 0.1875mol O
    By this point, I had already figured that this was likely going nowhere, as I still did not know the amount of KCl in KClO3 by itself, but rather just the moles of KCLO3+KCL without the oxygen.

    Next, is my teacher's solution.
    gKClO3 = 3.00gO x (1.22gKClO3/48.9gO) = 7.6625
    % = (7.6625/12.00) x 100% = 63.9%

    I understand the percent part, but what I do not understand is why 1.22gKClO3 is divided by 48.0gO. If I could have insight onto that, then I would be most appreciative.

    Thanks for reading my wall of text!
    Last edited: Jan 16, 2016
  2. jcsd
  3. Jan 16, 2016 #2


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    The teacher's solution glosses over some intermediate steps, so it's probably not as clear as it should be. Here are the steps I would take:

    1. Find the number of moles of oxygen atoms in 3g. (0.1875 moles as you calculated)
    2. Find how many moles of KClO3 can be created from 0.1875 moles of oxygen atoms.
    3. Find the mass of that number of moles of KClO3

    If you compress all of these 3 steps into one step, you get your teacher's solution.
  4. Jan 16, 2016 #3


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    I'm confused. Is the mass of the sample 16.00 g or 12.00 g?

    I'm confused again. You write 1.22 g on one line, and then you mention 122.6 g on the next line.

    You should proofread your posts carefully before submitting them.
  5. Jan 16, 2016 #4
    Could I multiply the mass of KClO3 by 0.1875 molesO to find the number of moles of KClO3? I'm not entirely sure
    You will have to forgive me. The information in the original question is the correct information. Anything else is my own error.
  6. Jan 16, 2016 #5
    I'm not entirely certain of how to find the number of moles KClO3 has using the number of moles of Oxygen atoms. Would multiplying 0.1875molO by the total mass of the mixture give me the moles of KClO? (Though I do not believe that to be correct...)
  7. Jan 16, 2016 #6


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    If you have 3 moles of oxygen atoms, how many moles of KClO3 do you have? Always look at the chemical formula.
  8. Jan 16, 2016 #7
    I have one mole of KClO3 then, with 1 mole of both K and Cl?
    (39+35.5)*0.1875 = 13.96875gKCl

    However, this number is more than double the number my teacher had gotten.
  9. Jan 16, 2016 #8


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    If you have one mole of KClO3, how many moles of oxygen do you have?
  10. Jan 16, 2016 #9
  11. Jan 16, 2016 #10


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    Now, work backwards. You have 0.1875 moles of oxygen. How many moles of KClO3 would 0.1875 moles of oxygen make?
  12. Jan 16, 2016 #11
    0.0625 moles?
  13. Jan 16, 2016 #12


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    Yes, this seems reasonable. What is the mass of this number of moles of KClO3 ?
  14. Jan 16, 2016 #13
    0.0625*122.55 = 7.659375g
    I see what I needed to do now. I guess I had forgotten that a mole is just a representative unit.
    % = (7.659375/12.00) x 100% = 63.828125 = 63.8%
    The number is slightly off, but that is just error.
    Thanks for your help!
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