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Homework Help: Given a mass, how much mass of this solid will dissolve in w

  1. Nov 25, 2017 #1
    1. The problem statement, all variables and given/known data
    If 0.025 g of Fe(OH)3 is added to 3.84 L of water, what mass will dissolve? Ksp is 2.8E-39.

    2. Relevant equations
    Ksp = [x][x]
    n= m/M

    3. The attempt at a solution
    I believe in this question that the starting mass is deemed irrelevant (although now i am starting to believe that is required to do something)

    So, Fe(OH)3 = Fe + 3OH, this means that our Ksp equation will be

    (x)(3x)^3 = 2.8E-39
    So we can simplify to 27x^4 = 2.8E-39, solve for x to get the concentration, which will yield 1.009E-10 mol/L.


    Now multiply by volume:
    1.009E-10 mol/L * 3.84 L = 3.875E-10 mol * 106.8670 g/mol = 4.14E-8 grams. This is what i am getting and this incorrect. Can someone explain why, and also can someone explain if we have to use the starting mass somehow.
     
  2. jcsd
  3. Nov 25, 2017 #2

    Borek

    User Avatar

    Staff: Mentor

    Think about concentration of OH-.
     
  4. Nov 25, 2017 #3
    ok so with the mass of the Fe(OH)3 we are given we can find the moles of it and turn it into concentration, is this what you are referring to?
     
  5. Nov 26, 2017 #4

    Borek

    User Avatar

    Staff: Mentor

    Yes, just remember to use the mass that you calculated from Ksp, not the original one given in the question.
     
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