Quick question on buffers

[aadharmg]

I can't seem to get my head around this question. I've got a few trains of thoughts but I don't know which one it is that the question is looking for. For 1 mark, 'explain why a mixture of sodium chloride and hydrochloric acid cannot form a buffer solution'.

[ateenytinybear]

my gut tells me its bc HCl is a strong acid, but im not suupppeerrrr sure

[RuiAce]

my gut tells me its bc HCl is a strong acid, but im not suupppeerrrr sure

Pretty much that causes the problem.

I can't seem to get my head around this question. I've got a few trains of thoughts but I don't know which one it is that the question is looking for. For 1 mark, 'explain why a mixture of sodium chloride and hydrochloric acid cannot form a buffer solution'.

Whilst you have mixed an acid with its corresponding conjugate base (HCl and the chloride ion), there's a problem in that HCl fully ionises. The purpose of mixing an acid with its conjugate base is to create a system that does not adjust significantly to small changes in pH (from adding small quantities of another acidic OR basic substance). Here, because HCl is a strong acid, the essentially inert chloride ion is not going to facilitate this - if you add more acid in there, the chloride ions are not going to go back into HCl.

[aadharmg]

Pretty much that causes the problem. Whilst you have mixed an acid with its corresponding conjugate base (HCl and the chloride ion), there's a problem in that HCl fully ionises. The purpose of mixing an acid with its conjugate base is to create a system that does not adjust significantly to small changes in pH (from adding small quantities of another acidic OR basic substance). Here, because HCl is a strong acid, the essentially inert chloride ion is not going to facilitate this - if you add more acid in there, the chloride ions are not going to go back into HCl.

Ohhhhh ok that makes a lot more sense. I was thinking about HCl being too strong of an acid and buffers needing to be weak acid/conjugate base pairs but I couldn't put it into words. Would I have to mention something along the lines of an equilibrium not existing between the acid and the conjugate base pair due to its strength, and hence creating an inability to reverse a change in pH? Also, how does the NaCl come into play in this question, considering it is just a neutral salt?

[RuiAce]

Ohhhhh ok that makes a lot more sense. I was thinking about HCl being too strong of an acid and buffers needing to be weak acid/conjugate base pairs but I couldn't put it into words. Would I have to mention something along the lines of an equilibrium not existing between the acid and the conjugate base pair due to its strength, and hence creating an inability to reverse a change in pH? Also, how does the NaCl come into play in this question, considering it is just a neutral salt?

Yeah. Equilibrium not existing is always important with strong acids.

Na+ is really just a spectator ion throughout it all. It's only Cl- that causes it to not work as a buffer