Understanding Acid-Base Buffers and Titration: Common Questions Answered

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In summary, the buffer needs to maintain an equal ratio of acid/salt, however as there is no resovoir of acid due to complete ionisation (therefore no equilibrium) any additional acid will essentialy just stay in the solution. 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.
  • #1
jsmith613
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Homework Statement


I have two questions

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

(b) why finding pKa 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 Ka expression and know that at the half-way point
[HA] = [H+] so Ka = [H+]
BUT WHY DOES IT ONLY WORK FOR WEAK ACIDS AND NOT WEAK BASES (with strong acid)?
 
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  • #2
Seems to me like both questions suggest things that are not true, which makes them impossible to answer.

jsmith613 said:
(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 pKa 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.
 
  • #3
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!
 

FAQ: Understanding Acid-Base Buffers and Titration: Common Questions Answered

1. What is an acid-base buffer?

An acid-base buffer is a solution that resists changes in pH when small amounts of acid or base are added to it. It is composed of a weak acid and its conjugate base or a weak base and its conjugate acid.

2. How do buffers maintain a constant pH?

Buffers maintain a constant pH by utilizing the principles of Le Chatelier's principle. When a small amount of acid is added to a buffer, the weak base component of the buffer reacts with it to form more of its conjugate acid, preventing a significant change in pH. Similarly, when a small amount of base is added, the weak acid component of the buffer reacts with it to form more of its conjugate base, again preventing a significant change in pH.

3. What is the purpose of a titration?

A titration is a laboratory technique used to determine the concentration of an unknown solution by reacting it with a known solution. It is commonly used in acid-base chemistry to determine the equivalence point, where the number of moles of acid is equal to the number of moles of base, and to calculate the concentration of the unknown solution.

4. How do you choose an appropriate buffer for a specific pH range?

The choice of an appropriate buffer for a specific pH range depends on the pKa of the weak acid or base component of the buffer. The desired pH of the solution should be within one unit above or below the pKa of the weak acid or base. The closer the desired pH is to the pKa, the more effective the buffer will be in maintaining a constant pH.

5. What is a buffer capacity?

Buffer capacity is a measure of the ability of a buffer to resist changes in pH when acid or base is added. It is dependent on the concentration of the buffer components and their pKa values. A buffer with a higher buffer capacity will be able to resist larger changes in pH compared to a buffer with a lower buffer capacity.

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