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Titration - solving for a one concentration knowing the other reactant

  1. Oct 6, 2007 #1
    1. The problem statement, all variables and given/known data

    A 10.02 mL solution of chemical A4+ is determined by precipitation titration with 40.07 mL of 0.03015 M titrant B3-. The reaction stoichiometry is given by

    3A4+ + 4B3- => A3B4 (s)
    For a log Ksp of -36.6, what is pB3- at the equivalence point?
    You must express your answer to two decimal places only in order to get the program to grade it properly!!!(Caution: Note the difference between Log X and pX, in terms of sign)

    2. Relevant equations

    -log[concentation]

    3. The attempt at a solution
    since its at equivalence point the concentration of [b3]and [a] would be the same.
    since ksp=[a]=-36.6
    In terms of x, wouldnt it be (3x^3)(4x^4) = -36.6? (6912x^7)=-36.6 = -1.1726 but then i cant take a -log of a negative number?
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
     
    Last edited: Oct 6, 2007
  2. jcsd
  3. Oct 6, 2007 #2
    what i tried doing now was

    since it said For a log Ksp of -36.6
    pKsp = -log[x]
    log[ksp] = -36.6
    2.51x10^-37?

    [3x]^3[4x]^4 = ksp
    6912x^7=2.51x10^-37
    x=1.67x10^-6

    -log 1.67x10^-6
    ph = 5.77

    but thats wrong too, the answer is 5.11

    so anyone have any ideas?
     
  4. Oct 7, 2007 #3
    correct

    Ofcourse it is wrong. The concentration of B^{4-} at equilibrium is [4x] and not x
     
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