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Homework Help: Henderson Hasselbalch equation! Where did I go wrong?

  1. Jan 18, 2009 #1
    1. The problem statement, all variables and given/known data

    Q) Calculate the volume of 0.1M HCl you would need to add to 200ml of 0.15M sodium acetate solution to obtain a buffer of pH 4.2.
    Ka = 1.74 * 10^-5 moldm-3

    2. Relevant equations

    pH = pKa + log [salt]/[acid]

    3. The attempt at a solution

    Log[salt]/[acid] = pH - pKa

    [salt]/[acid] = 0.275

    Equation: [salt] = 0.275 [acid]

    [salt] = 200*0.15/200+v

    [acid] = (0.1*V / 200+v) - (200*0.15/200+v)

    Substituting into above equation:

    200*0.15/200+v = 0.275(0.1*V/200+v) - 0.275(200*0.15/200+v)

    30 = 0.0275v - 8.25

    38.25= 0.0275v

    V = 38.25/0.0275v

    V = 1390 cm^3

    This is obviously wrong! Where did I go wrong?
  2. jcsd
  3. Jan 19, 2009 #2


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    Staff: Mentor

    Moles of salt + moles of acid should be constant.
  4. Jan 19, 2009 #3
    I still don't understand... could you explain a bit more please - sorry to bother you, I am just really desperate!
  5. Jan 19, 2009 #4


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    Staff: Mentor

    Take a look at the protonation reaction. When you add strong acid to acetate amount of acetic acid grows, but amount of acetate goes down.

    Could be you tried to account for that, but you did it wrong way.
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