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Homework Help: Chemistry Problem II

  1. Sep 26, 2007 #1
    My question is:

    Assuming the weight percent of CaCO3 is 50%, calculate the mass of unknown sample required to prepare a 15 mL solution that contains 0.005 M carbonate.

    Well i know that in the sample of unknown, there half of them are CaCO3. I don't know what do to next...........help plz

    Thank you
  2. jcsd
  3. Sep 27, 2007 #2
    if you mean the mass percent of CaCO3 is 50% in an unknown sample then, try getting to how many moles of CaCO3 you would need to make a .005 M solution of Carbonate in 15mL
  4. Sep 27, 2007 #3
    that means i would need:
    0.015 L x 0.005 mol/L = 7.5 E -5 moles of carbonate which means i neeed 7.5 E-5 moles of CaCO3

    then... ?
  5. Sep 28, 2007 #4
    keep going, how do you get mass (grams) from moles? a great equation to know is MM (molar mass) = g/mol

    then you have mass of CaCO3, if its 50% (or 1/2) of the mass (x) your unknown sample ....
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