A biochemist plans to make up a buffer solution with a pH of 5.000.
The biochemist adds solid sodium ethanoate, CH3COONa, to 400 cm3 of 0.200 mol dm–3
ethanoic acid.
Ka for ethanoic acid = 1.75 × 10–5 mol dm–3
Calculate the mass of sodium ethanoate that the biochemist needs to dissolve in the ethanoic
acid to prepare this buffer solution.
Assume that the volume of the solution remains constant at 400 cm3 on dissolving the sodium
ethanoate.

The mark scheme says that the answer is 11.48g but I don't understand how they got there and the calculations in the mark scheme are very confusing. Can someone help?
Original post by Coraw
A biochemist plans to make up a buffer solution with a pH of 5.000.
The biochemist adds solid sodium ethanoate, CH3COONa, to 400 cm3 of 0.200 mol dm–3
ethanoic acid.
Ka for ethanoic acid = 1.75 × 10–5 mol dm–3
Calculate the mass of sodium ethanoate that the biochemist needs to dissolve in the ethanoic
acid to prepare this buffer solution.
Assume that the volume of the solution remains constant at 400 cm3 on dissolving the sodium
ethanoate.

The mark scheme says that the answer is 11.48g but I don't understand how they got there and the calculations in the mark scheme are very confusing. Can someone help?

I've worked it out and got the correct answer, where have you gotten upto so far and i can try and explain it to you
Original post by rehman15
I've worked it out and got the correct answer, where have you gotten upto so far and i can try and explain it to you

All I've done is calculate the concentration of the buffer and write the equation for the buffer. I'm not sure where to go from there.
Original post by Coraw
All I've done is calculate the concentration of the buffer and write the equation for the buffer. I'm not sure where to go from there.

1. Rearrange the equation - [CH3COO-] = Ka[CH3COOH] / [H+]
2. [H+] = 10^-5 = 1 x 10-5
3. [CH3COO-] = 1.75 x 10-5 x 0.2 / 1 x 10-5 = 0.35 mol dm-3
4. 0.35 x 400 x 10-3 = 0.14 moles
5. 0.14 x 82 (Mr of sodium ethanoate) = 11.48g

i hope this makes a bit more sense, if you don't get anything i'll try and explain!
Original post by rehman15
1. Rearrange the equation - [CH3COO-] = Ka[CH3COOH] / [H+]
2. [H+] = 10^-5 = 1 x 10-5
3. [CH3COO-] = 1.75 x 10-5 x 0.2 / 1 x 10-5 = 0.35 mol dm-3
4. 0.35 x 400 x 10-3 = 0.14 moles
5. 0.14 x 82 (Mr of sodium ethanoate) = 11.48g

i hope this makes a bit more sense, if you don't get anything i'll try and explain!

Thank you so much! I was trying to figure out where the 0.35 came from but now i see it. Thanks a bunch
Original post by Coraw
Thank you so much! I was trying to figure out where the 0.35 came from but now i see it. Thanks a bunch

no probs!
A biochemist plans to make up a buffer solution with a pH of 5.000.
The biochemist adds solid sodium ethanoate, CH3COONa, to 400 cm3 of 0.200 mol dm–3
ethanoic acid.
Ka for ethanoic acid = 1.75 × 10–5 mol dm–3
Calculate the mass of sodium ethanoate that the biochemist needs to dissolve in the ethanoic
acid to prepare this buffer solution.
Assume that the volume of the solution remains constant at 400 cm3 on dissolving the sodium
ethanoate.

The mark scheme says that the answer is 11.48g but I don't understand how they got there and the calculations in the mark scheme are very confusing. Can someone help?

where did you get the markscheme from? i really need it
Original post by Mihnear72
where did you get the markscheme from? i really need it

its online just go on the OCR website I cant remember what paper it is but just find the one you want as long as its 2018 or below it will be on there if its the 2019 paper I have it but if you message me ill send it to you a few days later
Original post by rehman15
1. Rearrange the equation - [CH3COO-] = Ka[CH3COOH] / [H+]
2. [H+] = 10^-5 = 1 x 10-5
3. [CH3COO-] = 1.75 x 10-5 x 0.2 / 1 x 10-5 = 0.35 mol dm-3
4. 0.35 x 400 x 10-3 = 0.14 moles
5. 0.14 x 82 (Mr of sodium ethanoate) = 11.48g

i hope this makes a bit more sense, if you don't get anything i'll try and explain!

i know this is years ago, but my homework was on this, and honestly, I was close to breaking down. Thank you for this!!! I understood it so well!