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Homework Help: Chemistry Purity Problems with solutions help

  1. Oct 31, 2011 #1
    I have a test tomorrow and I thought I knew how to do these types of problems but my answers are all wrong.

    Here is the question:
    The solubility of CaSO4 at 25*C is 1.61x10^-2 mol/L. What mass will dissolve in 50.0 mL of water if the sample is only 20.0% pure?

    Heres what I did. First I found moles of CaSO4:
    n=1.61x10^-2 x 0.05L
    n=.00805 mol

    Then I converted moles of CaSO4 to mass (grams):
    m=0.00805mol x 136g/mol

    Then I did a conversion of 100% pure to 20%:
    1.0948g/100 = x/20
    (Cross multiply)
    100x = 21.896
    x = .219g
    Which means the sample of CaSO4 that is 20% pure should have a mass of .219g but according to the answers that is incorrect. Can someone tell me what I am doing wrong and how I can fix it. Thanks
    Last edited: Oct 31, 2011
  2. jcsd
  3. Oct 31, 2011 #2

    I like Serena

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    Homework Helper

    Welcome to PF, loz123! :smile:

    I interpret your problem statement as that you need to find a total mass of which 20% is actually CaSO4.
    As it is, 20% of .219 g is not 1.0948 g.
  4. Nov 1, 2011 #3


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    Staff: Mentor

    I find this question ambiguous - or intentionally tricky. You will dissolve 1.09g of CaSO4p/sub] in 50 mL of water regardless of the sample purity (unless sample contains other sulfates or other soluble calcium salt - but that's another problem).
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