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Homework Help: Dissolution stochiometry

  1. May 14, 2014 #1
    17. (20 marks) A 0.410g sample of sodium fluoride is dissolved in 25.00mL of water and is then reacted with 50.00mL of 0.500mol/L calcium nitrate solution. After the mixture is filtered, 0.225g of precipitate is recovered.
    a ) Write the net ionic equation for the reaction which occurs.
    b) Calculate the maximum mass of precipitate that could be formed (theoretical
    c) Calculate the percentage yield for this reaction.
    d) Calculate the concentrations of the three major ions in the final solution.

    For d) This website says :Mol Ca2+ remaining in solution: 0.025 mol Ca2+ initially - 4.88X10^-3 mol CaF2 formed = 0.0201 mol Ca2+ remaining / 0.075 L = 0.268 M Ca2+

    My textbook answer says the same thing.

    BUT WHY! That is the result of theoretical maximum yield 100%... I get 0.294M using 0.225g.
    Can you back me up?
  2. jcsd
  3. May 14, 2014 #2
    I feel dumb :S It looks really simple but I don't get that 0.268M! Why are they talking about full dissociation?!
  4. May 14, 2014 #3


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    Staff: Mentor

    It is a rather lousy question IMHO. I have no idea what is the "correct" answer here. That is, concentrations can be calculated from the solubility product, but that's an entirely different problem then.
  5. May 14, 2014 #4
    I think they waant to us to keep up with the 0.255g.. But apparently we are supposed to assume full dissociation!
  6. May 14, 2014 #5
    I agree. Bad formulation. And even their answer does not make sense since they ask for 0.225g dissociation.
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