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Homework Help: Finding a stoichiometric coefficient

  1. Sep 24, 2012 #1
    Hello, I am currently taking a college level chemistry class. I am struggling with this problem, any help would be greatly appreciated.

    1. The problem statement, all variables and given/known data
    We have a certain quantity of

    [tex] CuSO_{4} \cdot \gamma H_{2}O [/tex]

    If our sample is formed of (25,5% Cu), (12% S), (57,7% O) and (4.04% H), what is the value of gamma ?

    2. Relevant equations
    None given.

    3. The attempt at a solution
    I have determined the mass of 1mol of each element, the mass of 1mol of CuSO_4 (111.611g) and really, I'm stuck there.

    Please don't spoil the fun for me, just give me some hints.

    (edit) : I have done this much more :

    Mass of 1mol Cu = 63.546g

    (63.546g/111.611g)*100 = 56.94% Cu in CuSO_4

    56.94%/25.5% = 2.2328 (increment factor)

    So the mass of the hydrated CuSO_4 should be

    111.611g * 2,2328= 249.21g

    249.21g - 111.611g = 137.6g H2O

    and 137.6g/18(g/mol)= 7.6 mol H2O

    but it does not give me an integer...
    Last edited: Sep 24, 2012
  2. jcsd
  3. Sep 24, 2012 #2


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    Gold Member

    Try some Approximations in calculations. For example, Molar Mass of Cu is good as 63.5.
  4. Sep 24, 2012 #3
    thanks AGNuke, you mean that some approximations could get me closer to an integer coefficient? Do you think my attempt at a solution made sense?
    Last edited: Sep 24, 2012
  5. Sep 24, 2012 #4


    User Avatar
    Gold Member

    No. You can apply the percentage "after" you had taken the molar mass of all the compound (including the hydration) part in consideration.
    Take molar mass of compound as M[CuSO4]+γ[M(H2O)]

    Also, you took the molar mass of CuSO4 incorrect.

    And don't panic if you don't get answer in integer. Just try rounding them up or down.
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