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Yjee Feb 10th 2018 11:25 AM

Difficult buffer question...
 
Question -

You will prepare 2 buffer solutions, one which is pH 5.2 and one which is pH 8.8. Solutions available: 1M NH3, 1M NH4Cl, 1M ethanoic acid, 1M sodium ethanoate solution. Calculate how much of the above solutions you need to make 100cm3 of each buffer.

Use the equation pH = pKa - log [HA]/[A-]

Any help appreciated!

chemsurvival Apr 2nd 2018 12:23 PM

Hi Yjee,

The first step in your problem will be to decide on exactly which solutions to combine for each buffer. We usually prepare buffers by mixing solutions of conjugate acids and bases in differing proportions, so ammonia and ammonium mixtures make good buffers in certain situations while ethanoic acid and ethanoate might mix to make a good buffer in others.

Generally speaking, buffers are most effective when the pKa of the conjugate acid is close to the buffering pH. With that in mind and the following information, which two solutions do you think you need to use to make the pH 8.8 buffer? How about the pH 5.2 buffer?

ammonium pKa = 9.2

ethanoic acid pKa = 4.8

Eagerly awaiting your replies!

Professor Davis

Miya May 2nd 2018 08:50 PM

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