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Buffer and pH

  1. May 20, 2010 #1
    am really stuck on this problem, can't seem to get to the right answer

    1. The problem statement, all variables and given/known data


    How many mL of a 3.00 M HCO3- solution must be added to 200 mL of a 0.200 M CO32- solution to make a buffer with pH = 10.30?

    Answer. 13.96 mL

    2. Relevant equations

    pKa + log([base]/[acid])=pH

    HCO3(-) --> H+ + CO3 (2-)



    3. The attempt at a solution

    pKa + log([base]/[acid])=pH
    pH=10.3
    pKa= 5.6e-11 (this value is give to us in our book)

    0.048188 = log([base]/[acid])
    1.117=[base/acid]

    since the final volume cancels
    we simply just use the mole ratios
    let x be the mL of HCO3 - needed

    base= (0.20)(0.2)
    acid= 3x

    1.117=0.2(0.2)/3x

    x=11.933ml, which is way different to the actual answer



    would appreciated heaps if someone could help
    thanks
     
  2. jcsd
  3. May 20, 2010 #2

    Borek

    User Avatar

    Staff: Mentor

    Must be error in the answer (or some typo in the data), 11.93mL seems to be correct.

    --
    methods
     
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