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Homework Help: Percent by weight of substance in hydrate

  1. Sep 13, 2009 #1
    Q: Gypsum, used for wallboard, has the formula CaSO4 2H2O. (a) What is the percent by weight of water in this hydrate? (b) If 15.000 grams of this hydrate is heated, what mass of water would be lost?

    I'm thinking that I have to find out the mass of the CaSO4 and then the mass of 2H2O, then calculate the percentage of 2H2o to CaSO4. I just don't know how to find out the mass of either one..

    For part (b) i guess i assume that all water is lost in heating? Then use the percentage from (a) to calculate the water lost?

    thanks for any help,
  2. jcsd
  3. Sep 13, 2009 #2
    The percentage should be of H2O in the entire hydrate CaSO4·2H2O
    You use the atomic masses from the periodic table to find the masses...

    For (b) it sounds like you're on the right track.
  4. Sep 15, 2009 #3
    ok thanks
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