Chemistry · Grades 11–12
Buffers and acid–base ratios
Use a buffer approximation only where its assumptions fit.
Learn the idea
Buffer
A common buffer contains a weak acid and its conjugate base in appreciable amounts.
Role
A buffer resists small additions of acid or base; its capacity is finite.
Approximation
pH ≈ pKa + log₁₀([A⁻]/[HA]) in the appropriate dilute equilibrium approximation.
Limits
Large additions that consume a component require stoichiometric accounting before any buffer approximation.
Worked examples
Which pair can form a common acid buffer?
Both members of the conjugate pair must be present in useful amounts.
A weak acid and its conjugate base.
For a buffer with pKa 4.8 and [A⁻]/[HA] = 1, use the stated approximation to find pH.
log₁₀1 = 0, so pH ≈ pKa.
4.8
Check your understanding
For pKa 4.8 and [A⁻]/[HA] = 10, find the approximate pH.
Can a buffer resist an unlimited amount of added acid?
- Yes, by definition.
- Only if its name is short.
- No; its capacity is finite.
Why should amounts be recalculated after a significant addition before using the ratio expression?
Answers and marking notes
Answers and marking notes
- 5.8
log₁₀10 = 1, so pH ≈ 4.8 + 1.
- No; its capacity is finite.
A component can be substantially consumed.
- Added acid or base changes the quantities of the conjugate pair.
Initial concentrations need not remain the relevant ratio.
Further reading and review references
These are reference links, not copied exercises, institutional endorsement or proof of independent review.
Independent educational review required. Grade bands are approximate; these are not officially aligned curricula.