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% element by mass in soln.

  1. Dec 6, 2008 #1
    What is the percent CdSO4 by mass in a 1.0 molal aqueous solution?

    My attempt:

    CdSO4 is 208.47g/mol
    1.0 molal = 1 mol CdSO4/1kgH2O
    Converted mols to grams, and kg to g.
    208.47g Cd/1000g H2O = .20847
    .20847 x 100 to get % mass = 20.8%


    It is an old test question that I am going over for finals week. I keep coming up with a different answer than what was on my test. I keep getting the value of 20.8%, which the answer listed on my test is 17.2%. Did my teacher make a failure in grading or did I just do something differently the first time around? I cant find my original work.
     
  2. jcsd
  3. Dec 6, 2008 #2

    symbolipoint

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    You want the result as how? Percent weight per volume, or percent weight per weight? The rate might have been assumed to be straight weight or mass only:

    208 grams Cadmium Sulfate in 208+1000 grams SOLUTION. This perspective will affect the percentage very much.
     
  4. Dec 6, 2008 #3
    thank you i am retard
     
  5. Dec 6, 2008 #4

    symbolipoint

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    No, not really; you just forgot which perspective to use. Your teacher did not specify, so you should probably pick percent weight. Really, the instructions in the practice test should specify either %w/w, or %w/v.
     
  6. Dec 7, 2008 #5
    I think it is neither. It makes sense when you are calculating total % composition of Cadmium Sulphate that you would divide 208.47 by 1208.47 instead of just 1000. Because 1000g would just be the mass of the solvent not the soln.
     
  7. Dec 7, 2008 #6

    Borek

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    17.5% w/w or 20.8% w/v, note that the second number can't be calculated without knowing solution density.
     

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