If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

## Chemistry library

### Course: Chemistry library>Unit 14

Lesson 1: Buffer solutions

# Buffer capacity

The definition of buffer capacity, and an example showing why it depends on the absolute concentrations of the conjugate acid and base.

## Want to join the conversation?

• At how do you get the value that [A]/[HA]=1.82?
• in the previous video this process is explained. But [A]/[HA]= 10^pH-pKa. She gave an example PH=5, and then we were able to find the pKa from the Ka value she provided. Pka=4.75. So 10^5-4.75=1.82
• So the buffer capacity simply depends on the concentrations [AH] and [A-], higher both concentrations are, higher is the buffer capacity is?
• Why do you have to use the henderson-hasselbach equation instead of calculating pH normally? Does it have to do with how the buffers behave?
• For buffer#2, what would happen if the [HA] concentration had been exactly 0.04M? You would have been left with 0M left for [HA]. The ratio of [A^-] to [HA] would be 0.13/0 and since you can't divide by zero, what would the pH change then be?
• In these simple buffer problems we assume that ALL of the added H+ or OH- reacts with the buffer components [A-] or [HA]. Well, this is not that straightforward. Actually, this assumption works only when the amount of added H+/OH- is much smaller than the amount of buffer components.

I.e. is your set up with 4M of {HA} and 4M {OH-] added, our assumption that 100% of [OH-] reacts with [HA] will simply not work: not all of the [OH-] will react and the solution of the problem won't be that simple.

That's why in all the model buffer problems it's important to note that the about of acid/base added is small compared to the amount of buffer components.
• Can someone help me with this question. 'What is the maximum amount of the acid that can be added to a buffer made by the mixing of 0.35 moles of sodium hydrogen carbonate with 0.50 moles of sodium carbonate? How much base can be added before the pH will begin to show a significant change?'
• It depends on what you mean by "a significant change".
The buffer capacity is defined as the amount of acid or base you can add without changing the pH by more than 1 pH unit.
I will define "significant change" as 1 pH unit.
The equation is
HCO₃⁻ + H₂O ⇌ H₃O⁺ + CO₃²⁻
*(1)* pH = pKₐ + log([CO₃²⁻]/[HCO₃⁻]) = pKₐ + log(0.50/0.35) = pKₐ + 0.155
If we add x mol of base until the pH increases by 1 unit, we have
*(2)* pH + 1 = pKₐ + log[(0.50+x)/(0.35-x)]
Subtract (1) from (2)
1 = log[(0.50+x)/(0.35-x)] - 0.155
1.155 = log[(0.50+x)/(0.35-x)]
(0.50+x)/(0.35-x) = 10^1.155 = 14.29
0.50+x = 14.29(0.35-x) = 5.00 - 14.29x
15.29x = 4.50
x = 4.50/15.29 = 0.294
∴ We can add 0.294 mol of base before the pH changes by 1 unit.
Check:
New pH = pKₐ + log(0.794/0.056) = pKₐ + 1.152
Old pH = pKₐ + 0.155
ΔpH = 1.152 - 0.155 = 0.997 ≈ 1
• hello, Im wondering if it's possible to compare buffer capacity of two totally different buffers with different initial pH?
• Is there a way to say a buffer has x buffer capacity or everything is just relative?
(1 vote)
• Yes, buffer capacity is a specific number in moles.