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# Preparing a buffer solution watch

1. Would somebody be able to help me with the calculation for preparing a buffer solution?

I need to make 500ml of 50mM phosphate buffer solution using KH2PO4 (acid) and KOH (base) at pH 7.5.

I usually do it the stupid way by using a pH meter, but the experiment I have got to do rule out using a pH meter (the experiment is extremely sensitive to contamination from trace metals).

So I need to work out the exact amounts of the acid and base to weigh into a set volume of water. Does anybody know how to do this? If somebody could write down the workings, that would be awesome. Rep for anybody that can help!
2. buffers are made from the salt of a weak acid + a weak acid (or vice versa salt of weak base + weak base)

by the equilibrium law:

Ka=[H+][A-]/[HA]

take logs

logKa = log[H+] + log[A-]/[HA]

multiply through by -1

pKa = pH - log[A-]/[HA]

so if you know the pKa of the weak acid and the intended pH

yo can work out possible combinations of [A-] and [HA] from:

pH - pKa = log[A-]/[HA]

so now the fun bit is reactiong the KOH with the KH2PO4 in the correct quantities to obtain the correct final proportions...

as you haven't provided the pKa value or the original acid abnd base concentrations I can't do it for you
3. (Original post by charco)
buffers are made from the salt of a weak acid + a weak acid (or vice versa salt of weak base + weak base)

by the equilibrium law:

Ka=[H+][A-]/[HA]

take logs

logKa = log[H+] + log[A-]/[HA]

multiply through by -1

pKa = pH - log[A-]/[HA]

so if you know the pKa of the weak acid and the intended pH

yo can work out possible combinations of [A-] and [HA] from:

pH - pKa = log[A-]/[HA]

so now the fun bit is reactiong the KOH with the KH2PO4 in the correct quantities to obtain the correct final proportions...

as you haven't provided the pKa value or the original acid abnd base concentrations I can't do it for you
How do I get the pKa values? Sorry for being stupid! And thanks for your reply
4. you can't work them out unless you have the concentration and pH of the weak acid/base in question. Look up the pKa in a databook.

If you have the pH and concentration then use the equation in the first post:

Ka=[H+][A-]/[HA]

and as [H+]=[A-] then Ka=[H+]^2/[HA]

you know [HA] (its the concentration of the weak acid)
you can get [H+] from the pH
so you can work out pKa
5. Meh, I think i'll get a lecturer to explain it to me. Thanks anyway.
6. Some pKa and pKb values. Phosphoric acid included.

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