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# Buffers -URGENT watch

1. A scientist wished to prepare a pH 4.94 buffer solution using solutions of 0.1M acetic acid and 0.1M sodium acetate. How much sodium acetate solution should she add to 200mL 0.1M acetic acid to make the buffer solution?. The pKa of acetic acid = 4.76.

Could some one guide me through this please? Thank you
2. Just wondering did the question specifically say 0.1M of sodium acetate? If it did then idk but if it didn't this makes the question more like the ones I've done in chem class
3. (Original post by j582)
A scientist wished to prepare a pH 4.94 buffer solution using solutions of 0.1M acetic acid and 0.1M sodium acetate. How much sodium acetate solution should she add to 200mL 0.1M acetic acid to make the buffer solution?. The pKa of acetic acid = 4.76.

Could some one guide me through this please? Thank you
The first thing you need to calculate is the target concentration ratio of acid and salt.

You do this using the acid dissociation equation and the pH

pH 4.94 corresponds to [H+] = 1.148 x 10-5

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

1.738 x 10-5 = 1.148 x 10-5[A-]/[HA]

Hence the ratio of salt to acid must be [A-]/[HA] = 1.514

You now that the volume of acertic acid is 200ml and the molarity = 0.1 so the total moles of acid = 0.2 x 0.1 = 0.02

As both the acid and salt are in the same final volume you can now deal directly in moles.

To get the moles of salt use the relationship [A-]/[HA] = 1.514

Hence [A-] = 1.514 x 0.02 = 0.03028 moles of salt required.

You are told that the molarity of the salt = 0.1, therefore you need to use:

moles = molarity x volume

volume needed = 0.03028/0.1 = 0.3028

303 ml (3 sig figs)

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