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HELP!! Buffer preparation calculation!

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    What volume of 0.2M NaOH is required to prepare 200ml of a 0.2M buffer solution of pH 4 (pKa of acid is 3.5)?


    Ive started off using ph=pka+log(b/a) then im not sure what to do..

    Thanks.
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    (Original post by shakey1)
    What volume of 0.2M NaOH is required to prepare 200ml of a 0.2M buffer solution of pH 4 (pKa of acid is 3.5)?


    Ive started off using ph=pka+log(b/a) then im not sure what to do..

    Thanks.
    LOL i am stuck on this exact same question....
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    (Original post by mpatel90)
    LOL i am stuck on this exact same question....
    Could you link me to the full question ?
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    hey think I have managed to figure it out - but please double check...

    Q) What Volume of NaOH is required to prepare 200ml of a 0.2M buffer with a pH of 4 and pKa of acid = 3.5?

    Step 1: work out ratio of acid and base in buffer using Henderson Hasselbach Eqn


    [H+] = Ka (a/b)

    H+ = antilog pH = 1x10-4
    Ka = antilog pKa = 3.16 x 10-4

    putting that into above equation you get a/b = 0.3162

    Step 2: Work out concentrations of acid and base in buffer

    the total concentration of buffer is 0.2M therefore a+b= 0.2M
    so you can set up simultaneous equations to work out a and b

    a=0.2 - b and a= 0.3162b

    solving these gives you a = 0.0480 M and b = 0.1519 M

    Step 3: Working out volume of base required

    You know that the total volume of buffer is 200ml

    therefore the amount of b is in 200ml
    i.e. 0.1519 molar in 200ml
    so what volume of 200ml would have 0.2M in?

    (0.1519/0.2) X 200ml = 151.9 ml

    therefore the volume of NaOH reqd to make this buffer is 151.9ml
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    (Original post by mpatel90)
    x
    Yes, that is correct.

    Which exam board is this ?
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    (Original post by Ari Ben Canaan)
    Yes, that is correct.

    Which exam board is this ?
    Thanks - umm the question isnt from A Level stuff, I got it from a set of questions given to us by my lecturer at Uni (Ist year pharmacy...)
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    (Original post by mpatel90)
    Thanks - umm the question isnt from A Level stuff, I got it from a set of questions given to us by my lecturer at Uni (Ist year pharmacy...)
    OHHH !! No wonder this seemed a little different from the questions I'm used to
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    (Original post by mpatel90)
    hey think I have managed to figure it out - but please double check...

    Step 3: Working out volume of base required

    You know that the total volume of buffer is 200ml

    therefore the amount of b is in 200ml
    i.e. 0.1519 molar in 200ml
    so what volume of 200ml would have 0.2M in?

    (0.1519/0.2) X 200ml = 151.9 ml

    therefore the volume of NaOH reqd to make this buffer is 151.9ml
    It would be appreciated if you explain the last part a bit more

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