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I need help with this Henderson Hasselbalch equation please!

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  • I need help with this Henderson Hasselbalch equation please!

    The method must be wrong, although I used the same one when calculating the unknown volume for an acid and it was the right answer, but this is obviously not. Here is what i did:

    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.
    (Note: CH3COO- + H+ = CH3COOH)
    Ka = 1.74 * 10^-5 moldm-3

    pKa= 4.76

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

    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

    Okay so obviously that is not right!!! Where didi I go wrong??? Is it the bold part that is wrong?

  • #2
    Re: I need help with this Henderson Hasselbalch equation please!

    Hi

    What University are you at?

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