Understanding Acid-Base Buffers and Titration: Common Questions Answered

[jsmith613]

Homework Statement

I have two questions

(a) why can't strong acids / bases act as buffers

(b) why finding pK_a from an acid-base titration why does it ONLY work when we have a strong base-weak acid

Homework Equations

The Attempt at a Solution

Ok so for (a) here is what I think
- a buffer needs to maintain an equal ratio of acid/salt
- however as there is no resovoir of acid due to complete ionisation (therefore no equlibrium) any additional acid will essentialy just stay in the solution

for (b)
- I know the K_a expression and know that at the half-way point
[HA] = [H+] so K_a = [H+]
BUT WHY DOES IT ONLY WORK FOR WEAK ACIDS AND NOT WEAK BASES (with strong acid)?

 

[Borek]

Seems to me like both questions suggest things that are not true, which makes them impossible to answer.

(a) why can't strong acids / bases act as buffers

What is a buffer? If it is a solution that resists changes of pH when small amount of acid/base are added, solutions of strong acids and bases work this way - just at high (low) pH. So At pH 1.0 0.1M solution of HCl behaves as buffer, keeping pH stable. Compare buffer capacity.

When pH is somewhere between 2 and 12 we get higher buffer capacity using solutions containing both an acid and a conjugate base. So yes, in this pH range strong acids and bases will not work as buffers. But weak acids and bases will not work outside, so if you want to be able to buffer pH of the solutions regardless of pH, you can't say "strong acids and bases don't show a buffering effect".

why finding pK_a from an acid-base titration why does it ONLY work when we have a strong base-weak acid

The only thing I can think of is that for weak bases titrated with a strong acid we can easily find pKb, not pKa. But then we are just a subtraction away from knowing pKa, which means question either tries to be tricky, or doesn't make sense.

 

[jsmith613]

for (a) I see your point

for (b) I made a stupid assumption - my book only mentions Ka which is why I made the assumption.

Thanks so much for your help Borek!
Happy new year!