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Mass Of Oxygen

  1. Nov 20, 2015 #1
    1. The problem statement, all variables and given/known data
    What would be the mass of oxygen in 2.5 x 102g of CaCO3?

    Given:
    mCaCO3=250g
    M=100.09g/mol
    n=2.498mol

    2. Relevant equations
    ?

    3. The attempt at a solution

    mass of O=250g/3 atoms of O
    mass of O=83g

    Is that correct?
    Bc if it is can someone please explain why it's correct.
     
  2. jcsd
  3. Nov 20, 2015 #2

    SteamKing

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    Remember, 250 g of CaCO3 contains more than just Oxygen. What about the Calcium and the Carbon in the substance? What happened to them?
     
  4. Nov 20, 2015 #3
    So oxygen wouldn't be 83g?
     
  5. Nov 20, 2015 #4

    SteamKing

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    Honestly, what do you think?

    You can't divide the mass of a substance by the number of atoms of one constituent to obtain the mass of that constituent if other kinds of atoms are present. That should be understood, whether you're taking chemistry or not.
     
  6. Nov 20, 2015 #5
    Not to put the blame on others someone showed me how to do it that way which made me confused.

    So I'm assuming the final answer is 119.9g of Oxygen.
     
  7. Nov 20, 2015 #6

    SteamKing

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    And how did you arrive at that figure?

    Please show your work.
     
  8. Nov 20, 2015 #7
    Given:
    mCaCO3=250g
    MCaCO3=100.09g/mol
    nCaCO3=2.498mol

    Required:
    mO=?

    Solution (I tried two methods):

    Method #1:

    %decimal= 16(3)/100.09g/mol= 0.4796

    (0.4796)(250g of CaCO3)= 119.9g of O

    Method #2:

    250gCaCO3/100.09g/mol=2.498mol

    2.498mol x 3 molO = 7.494molO

    7.494molO x 16gO = 119.9 g of o
     
  9. Nov 20, 2015 #8

    SteamKing

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    Method 1:
    0.4796 is the fraction of the mass of CaCO3 which is composed of Oxygen. This fraction remains the same regardless of the how much CaCO3 you are given.

    0.4796 is also called the ratio of the mass of Oxygen to the mass of CaCO3.

    Now, do you see why your original method of calculation in the OP was faulty?

    Method 2:
    There are 2.498 mol of CaCO3 in 250 g of the substance. There are 3 atoms (not moles) in each molecule of CaCO3 .

    That's why there are 2.498 mol × 3 atoms of O / molecule = 7.494 moles of Oxygen in 250 g of CaCO3
     
  10. Nov 21, 2015 #9
    I now see why the original calculation was incorrect.
     
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