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Homework Help: A piece of iron weighing 85.65 grams was burned in air.

  1. Oct 18, 2009 #1


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    1. The problem statement, all variables and given/known data
    A piece of iron weighing 85.65 grams was burned in air. The mass of the iron oxide produced was 118.37g.

    1. Use the law of conservation of mass to calculuate the mass of oxygen that reacted with iron.

    2. use the molar mass of oxygen to calculate the number of moles of oxygen atoms in the product.

    3. Use the molar mass of Fe to calculate the Moles of Fe used.

    4. What is the emperical formula of Iron oxide?

    2. Relevant equations

    Molar mass of O: 16.00
    Molar mass of Fe:55.85

    3. The attempt at a solution
    1. My answer : 32.72g of O2.
    I subtracted 85.65 from 118.37.

    2. 2.045 moles of O.

    I divided 32.72 by 16.

    3. 1.534 moles.

    I divided the grams by the molar mass of Fe.

    4. F3O4

    Is there any error in my calculations? Does the emperical formula always come out even?
    For example: I got Feo1.33 and just multiplied by 3, but I didn't exactly get 4 for O, I got 3.99, is that okay to do?
  2. jcsd
  3. Oct 18, 2009 #2


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    Staff: Mentor

    Re: stoichometry

    Looks OK to me. Coefficients in empirical formulas are almost never exactly integers, that's due to experimental errors.

  4. Oct 18, 2009 #3


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    Homework Helper
    Education Advisor
    Gold Member

    Re: stoichometry

    What about how practical is the question? Does the oxide diffuse into the unreacted metal and does the oxygen diffuse into the superficial iron oxide? The original question is just an academic exercise, but still is the actual question practical?
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