[cookie13]

Hi there

I was wondering if anyone could help me with this buffers question ...
For which of these liquids is the pH likely to change least if a splash of concentrated sulphuric acid is added? Justify your answer:

a) 0.1M hydrochloric acid solution
b) 0.08M ethanoic acid/0.02M potassium ethanoate mixture
c) 0.8M ammonia/0.2M potassium ethanoate mixture
d) pure water

a is a strong acid and therefore it will have no buffering capacity ... and pure water (d) has no buffering capacity either ... Which leaves b and c...

The answer is (c) but how do you come to this answer?

There's a similar question:
"Which of these liquids will have the least change in pH if a sodium hydroxide pellet were to drop into a container of the liquid? Justify your answer:

a) pure water
b) 0.01 M ammonium chloride/0.01M ammonia mixture
c) 0.05M ammonium chloride/0.05 M ammonia mixture
d) 0.01M ammonium chloride/0.1M ammonia mixture

(Correct answer is c)


Any help will be much appreciated

[Borek]

Take a look at my page here:

http://www.chembuddy.com/?left=pH-calculation&right=pH-buffer-capacity

[Beatle]

I have a feeling that the first so called answer is wrong. I buffering system is usually classified by the addition of a weak acid(acetic acid-ethanoic acid etc) and a salt of this weak acid - potassium acetate or ethanoate ( another name for acetic as an anion) I dont see how ammonia which if it is with a potassium ethanoate then it is in solution hence ammonia would be in the form of ammonium hydroxide.Perhaps it is the hydroxide that would react as a stronger base compared to the ethanoate which is also a strong base- conjugative base of a weak acid must be strong. I am as confused as you perhaps. I would go with my intiall assessment.
Part 2 appears that it is the amount that is what is important hence can result in less effect due to the amount of molar concentration. It might also be important to find out if ammonium chloride acts as an acid.

[Borek]

c is not necesarilly a classic buffer solution, but the question asks about the smallest pH change. You have concentrated solution of two bases - one being ammonia, second being acetate ion. It is like the beginning of the titration curve - pH changes are very slow.

[Jed Clampett]

Seems to me young 'un that the answer to the first question is a)

If yer already have a pH of 1, how much change will yer git with a splash o' sulphuric acid?

Hain't hardly none, I figger....

[Borek]

Could be a, although to decide it will require calculations. Ammonium acetate is much more concentrated, so buffering effect can be more prominent. Using formula for buffer capacity (linked to earlier) should give definite answer.

If yer already have a pH of 1, how much change will yer git with a splash o' sulphuric acid?

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