F325 jun 2011 buffers question Watch

EveryNameIsTaken
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Does anyone know how to answer part e of this question? I thought that any weak acid could be used for any acidic pH buffer?

http://www.ocr.org.uk/Images/65869-q...d-elements.pdf
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KanKan
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It can. I did that question yesterday, you can choose any of the acids. Ugh, I hate questions like these.
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Borek
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Not exactly. You want an acid that has pKa close to required pH. The closer, the better the buffer.
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KanKan
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(Original post by Borek)
Not exactly. You want an acid that has pKa close to required pH. The closer, the better the buffer.
The mark scheme does allow you to use any:

http://www.ocr.org.uk/Images/60302-m...ments-june.pdf
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Adil96m
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(Original post by KanKan)
The mark scheme does allow you to use any:

http://www.ocr.org.uk/Images/60302-m...ments-june.pdf
You don't get the first mark, but if you did everything else right you would get the rest of the marks

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Pigster
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The M/S may allow the usage of any combination, but you can only score full marks by choosing lactic/lactate, which as Borek points out, is the combination with the pKa value closest to the desired pH of the buffer.
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KanKan
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(Original post by Pigster)
The M/S may allow the usage of any combination, but you can only score full marks by choosing lactic/lactate, which as Borek points out, is the combination with the pKa value closest to the desired pH of the buffer.
Oh okay, but why is this?
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Pigster
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If you prepare a buffer with a pH = pKa then it can act as a buffer equally well when acting acid or alkali. The further the pH is from the pKa value, the more HA or A- you have to add to get it to that pH and hence the buffer proportionally get worse in one of the directions. I'm off to school, now, so don't have time to explain more fully. I'll check in later to see if you follow.
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Borek
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Compare http://www.chembuddy.com/?left=buffe...uffer-capacity
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KanKan
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I don't get why:

(Original post by Pigster)
hence the buffer proportionally get worse in one of the directions.
Why does it matter if both A- and H+ are in excess?
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Pigster
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Assuming your buffer contained 1 mol of HA and 1 mol of A-, you could add a lot of either acid or alkali to the buffer and the pH wouldn't change much. It would be equally good at buffering in both directions.

Now assume that the buffer had to be made up at a different pH and the ratio was 0.1 mol HA and 1 mol A-. Now, you can still add a lot of (as much as last time) acid and the pH wouldn't change much, but you can add comparatively much less alkali and expect the pH to remain similar - you are quickly going to run out of HA quicker.
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KanKan
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(Original post by Pigster)
Assuming your buffer contained 1 mol of HA and 1 mol of A-, you could add a lot of either acid or alkali to the buffer and the pH wouldn't change much. It would be equally good at buffering in both directions.

Now assume that the buffer had to be made up at a different pH and the ratio was 0.1 mol HA and 1 mol A-. Now, you can still add a lot of (as much as last time) acid and the pH wouldn't change much, but you can add comparatively much less alkali and expect the pH to remain similar - you are quickly going to run out of HA quicker.
Ahh that makes sense! Its because they wanted a specific pH. Thanks !
But why does a similar pKa = pH? Are pKa and pH not radically different?
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Pigster
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Ka = [H+] x [A-]/[HA]

If:

[A-] = [HA] e.g. if you added 1 mol of HA to 1 mol of A-Na+, or if you added 1 mol of NaOH to 2 mol of HA (since 1 mol of HA would react to form 1 mol of A-)

Then:

Ka = [H+] x 1

Then take the -LOG of both sides:

-LOG Ka = -LOG [H+]
i.e.
pKa = pH
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