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Chemistry Homework - the empirical formula of crocetin

  1. Nov 18, 2013 #1
    Chemistry Homework -- the empirical formula of crocetin

    1. The problem statement, all variables and given/known data

    Crocetin consists of elements carbon, hydrogen and oxygen. Determine the empirical formula of crocetin, if 1.00g of crocetin forms 2.68g of carbon dioxide and 0.657g of water when it undergoes complete combustion.

    2. Relevant equations

    n=m/M,

    3. The attempt at a solution
    I have no idea how to do this question I tried for 30 min and I cant get anything.
     
  2. jcsd
  3. Nov 18, 2013 #2
    Use the formulas of carbon dioxide and water to find the number of moles of each element produced. This will be equivalent to the number of moles before the reaction. The ratio of these numbers to each other will tell you the empirical formula.

    To explain it in more simple terms, water can be broken down into moles and so can carbon dioxide. Since there are two hydrogen atoms and one oxygen atom in a molecule of water, the number of moles in water will be equal to the number of moles of oxygen (because for 1 molecule of water there is 1 oxygen atom) and the number of moles of hydrogen will be twice the number of moles of water (because there are 2 hydrogen molecules in water).

    The same goes for carbon dioxide. For 1 mole of carbon dioxide, there is one mole of carbon and two moles of oxygen. Take the number of moles of oxygen from each product and add them together to get the total number of moles of oxygen that you had before the reaction. The number of moles of each atom is conserved, so if you find how many moles of each are present after the reaction, you know how many moles there were before. Once you get how many moles of each element, you can find the ratio between them, which will give you the empirical formula.
     
  4. Nov 19, 2013 #3

    Borek

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    Staff: Mentor

    Google for empirical formula, plenty of tutorials on the web.
     
  5. Nov 19, 2013 #4

    Borek

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    Staff: Mentor

    So far so good.

    Huh?

    I assume you mean "number of moles of water"?

    Beware - two moles of oxygen can mean either two moles of oxygen atoms or two moles of oxygen molecules. These are two different things and you should be paying attention to you wording to avoid ambiguity.

    Yes, but no. Yes, it will tell you how much oxygen was present before the reaction, but as it was a combustion reaction some of the oxygen came not from the molecule itself. Correct approach is to calculate mass of carbon and hydrogen in the original sample and to subtract these numbers from the original sample mass - whatever is left was most likely oxygen (unless the original compound contained also other, not mentioned elements).
     
  6. Nov 24, 2013 #5
    Still Dont get iit

    Ok I have a test tomorrow and I still dont get this question.
     
  7. Nov 24, 2013 #6
    Blood hemoglobin contains 0.33% iron. Assuming that there are two atoms of iron per molecule of hemoglobin, calculate the approximate molecular weight of hemoglobin.

    I dont get even how to start a question like this can someone tell me what their answer is and how they did it please.
     
  8. Nov 25, 2013 #7

    Borek

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    Staff: Mentor

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