You are Here: Home

# HELP!! Buffer preparation calculation!

Announcements Posted on
Talking about ISA/EMPA specifics is against our guidelines - read more here 28-04-2016
1. 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.
2. (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....
3. (Original post by mpatel90)
LOL i am stuck on this exact same question....
Could you link me to the full question ?
4. 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
5. (Original post by mpatel90)
x
Yes, that is correct.

Which exam board is this ?
6. (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...)
7. (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
8. (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

## Register

Thanks for posting! You just need to create an account in order to submit the post
1. this can't be left blank
2. this can't be left blank
3. this can't be left blank

6 characters or longer with both numbers and letters is safer

4. this can't be left empty
1. Oops, you need to agree to our Ts&Cs to register

Updated: May 5, 2012
TSR Support Team

We have a brilliant team of more than 60 Support Team members looking after discussions on The Student Room, helping to make it a fun, safe and useful place to hang out.

This forum is supported by:
Today on TSR

### How to predict exam questions

No crystal ball required