This cheat sheet covers how buffers resist changes in pH and how the Henderson-Hasselbalch equation connects pH to the amounts of weak acid and conjugate base. Students need this reference because buffer problems often combine equilibrium ideas, logarithms, mole ratios, and stoichiometry. It is especially useful for titration questions, biology-related pH systems, and AP or honors chemistry review.
The key formula is , where is the weak acid and is its conjugate base. A buffer works best when , so . Buffer capacity increases when the total concentration of and increases, and it is greatest when the acid-base amounts are similar.
Key Facts
- The Henderson-Hasselbalch equation is .
- For a base buffer pair, the related form is .
- The relationship between acid strength and is .
- A buffer is most effective when , which occurs when .
- A useful buffer range is usually , because the ratio stays between about and .
- When strong acid is added, it reacts mainly as before the Henderson-Hasselbalch equation is used.
- When strong base is added, it reacts mainly as before the Henderson-Hasselbalch equation is used.
- Buffer capacity increases as the total buffer concentration increases, even if the ratio stays the same.
Vocabulary
- Buffer
- A solution that resists large changes in when small amounts of strong acid or strong base are added.
- Weak acid
- An acid such as that only partially ionizes in water and establishes an equilibrium with and .
- Conjugate base
- The particle formed when the weak acid loses one proton.
- A logarithmic measure of acid strength defined by .
- Buffer capacity
- The amount of strong acid or strong base a buffer can neutralize before its changes significantly.
- Henderson-Hasselbalch equation
- An equation that estimates buffer using .
Common Mistakes to Avoid
- Using concentrations before neutralization, which is wrong when strong acid or strong base has been added. First update the moles of and using the reaction stoichiometry.
- Flipping the ratio in the Henderson-Hasselbalch equation, which gives the wrong sign for the logarithm. The acid form goes in the denominator as .
- Assuming dilution changes buffer , which is usually wrong if both and are diluted by the same factor. The ratio stays the same.
- Choosing a buffer with far from the target , which makes the buffer weak in that range. A good buffer usually has within about unit of the desired .
- Confusing buffer capacity with buffer , which ignores total amount. Two buffers can have the same but different capacities if one has larger .
Practice Questions
- 1 A buffer contains of and of in . If , find the .
- 2 A buffer has of and of . After of is added, what ratio should be used in the Henderson-Hasselbalch equation?
- 3 A chemist needs a buffer at . Which acid is the better choice, one with or one with ? Explain briefly.
- 4 Two buffers have the same ratio , but Buffer has total buffer concentration and Buffer has total buffer concentration. Which has greater buffer capacity, and why?
Understanding Buffer Capacity & Henderson-Hasselbalch Examples
At the particle level, a buffer contains two substances that can handle opposite disturbances. The base member of the pair removes added hydrogen ions. The acid member removes added hydroxide ions by donating hydrogen ions.
These reactions are fast compared with the small equilibrium shift that would otherwise cause a large pH change. The buffer does not keep pH perfectly fixed.
It reduces the size of the change until one member of the pair is nearly used up. This is why a buffer can fail suddenly after enough strong acid or strong base has been added.
Most calculation errors happen because students use the acid base ratio too early. First treat the added strong reagent as a complete reaction. If strong acid is added, subtract its moles from the conjugate base moles and add the same number of moles to the weak acid.
If strong base is added, subtract its moles from the weak acid and add them to the conjugate base. Only after this mole bookkeeping should the Henderson Hasselbalch relationship be applied.
When both buffer components are in the same final volume, their concentration ratio equals their mole ratio. This shortcut is useful, but it only works because the common volume cancels from the ratio.
Capacity depends on amount, not just balance. Two buffer mixtures can have the same pH because they have the same acid to base ratio, yet one can resist far more added reagent. For example, a dilute mixture may contain only a small number of moles of each component.
A concentrated mixture with the same ratio has many more particles available to react. Dilution with pure water usually leaves the ratio nearly unchanged, so the estimated pH changes very little.
However, dilution lowers capacity because fewer buffer particles are present in each liter. At very high dilution, the approximation becomes less reliable because water and ordinary acid equilibrium effects can no longer be ignored.
Choosing a buffer for a real system starts with the target pH. A useful acid base pair has a pKa close to that target, since both members then remain present in meaningful amounts. Blood, cells, food products, medicines, soil samples, and swimming pools all depend on controlled pH, though their buffer chemicals differ.
In a titration, the half neutralization point is especially important. At that stage, equal moles of weak acid and conjugate base are present, so pH matches pKa. When studying, label every substance as weak acid, conjugate base, strong acid, or strong base before doing arithmetic.
Then track moles with units and check whether the final ratio makes chemical sense. Added acid should make the mixture more acidic, while added base should make it more basic.