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HELP!! Buffer preparation calculation!

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.
Reply 1
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....:frown:
Original post by mpatel90
LOL i am stuck on this exact same question....:frown:


Could you link me to the full question ?
(edited 11 years ago)
Reply 3
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
Original post by mpatel90
x


Yes, that is correct.

Which exam board is this ?
Reply 5
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 :smile: (Ist year pharmacy...)
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 :smile: (Ist year pharmacy...)


OHHH !! No wonder this seemed a little different from the questions I'm used to :tongue:
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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