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Homework Help: Percent Composition

  1. Dec 11, 2014 #1
    1. The problem statement, all variables and given/known data
    An alloy of copper and silver weighing 0.5000 grams is dissolved in HNO3 and treated with H2S to precipitate both CuS and Ag2S. It is found that the solid sulphides weigh 0.7300 grams. Calculate the percentage of Ag in the alloy. (12.9 %)

    2Ag+(aq) + H2S(aq)→ Ag2S(s) + 2H+(aq)
    Cu+2(aq) + H2S(aq)→ CuS(s) + 2H+(aq)

    2. Relevant equations
    mass of Ag/mass of alloy = % of Ag

    3. The attempt at a solution
    I'm not too sure how to start this question…

    So far, I have

    0.7300 g Ag2S x 1 mol / 215.74 g Ag2
    = 2.945 x 10-3 mol Ag2S

    2.945 x 10-3 mol Ag2S x 215.74 g Ag2/ 1 mol Ag2S
    = 0.6353 g Ag2

    I'm not sure if I'm on the right track here?
  2. jcsd
  3. Dec 11, 2014 #2


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    You have a system of two equations in two unknowns.
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