Chem a level buffers question

State how a buffer solution can be made from solutions of potassium hydroxide and ethanoic acid.
Give an equation for the reaction between potassium hydroxide and ethanoic acid. State how this buffer solution resists changes in pH when a small amount of acid is added.

How buffer solution resists pH change
[3 marks]


MS: CH3COO– (from salt) reacts with (added) acid/H+

I said this, but I also said that because CH3COO- concentration is very low, the OH- ions from the KOH could react with H+ as well, is this idea wrong?..if not why does the markscheme only accept the first statement?


and also another question: when we say all ion concentrations to 1moldm-3 what ions are we regarding that for. E.g. I know for H2SO4 you would normally use 0.5M but surely you could say that the SO4- ions are not 1M anymore...?

In a buffer the concentration of salt CH3COO- isn’t very low. It would be in a weak acid but that isn’t a buffer. Ideally the concentration of acid and salt in a buffer are the same so [HA]/[A-] = 1.
as for all ions at 1 moldm-3 it doesn’t include spectator ions just what’s reacting.

No. The KOH is not part of the buffer system and is not providing OH- ions in the buffer. All the OH- ions from the KOH are used up reacting with the CH3COOH to make the salt, CH3COO-.
A buffer is a mixture of a weak acid and its salt. In this case we are partially neutralising the weak acid with a strong base to make the salt rather than adding the salt directly to the weak acid.