To calculate the pH of an acidic buffer after adding a small amount of strong acid, first subtract the moles of acid added from the moles of conjugate base. Add that same amount to the moles of weak acid formed. Use the resulting amounts to estimate pH.
The 2027 H2 Chemistry 9476 syllabus includes calculating buffer pH from given data. This example shows how to account for the reaction before using the acid equilibrium and how to tell when the mixture needs a different calculation.
- Write the reaction between H⁺ and the conjugate base.
- Calculate the moles of each buffer component after reaction.
- Use the final base-to-acid ratio in the pH calculation.
- Check whether any conjugate base remains.
CHAPTER 1 OF 4 · Back to contents
Separate reaction amounts from equilibrium
For an acidic buffer made from HA and A⁻, the added H⁺ reacts with A⁻ to form HA. Each mole of H⁺ consumes one mole of A⁻ and forms one mole of HA. If the question supplies a concentration and a volume, convert the volume to dm³ when using a concentration in mol dm⁻³ to calculate moles.
Once the strong acid has reacted, the weak-acid equilibrium determines the pH. The buffer approximation gives pH ≈ pKₐ + log₁₀([A⁻]/[HA]). The final volume cancels from this ratio because both concentrations describe the same mixture.
CHAPTER 2 OF 4 · Back to contents
Worked example | Updating the buffer ratio
A buffer contains 0.020 mol of HA and 0.020 mol of A⁻. The pKₐ of HA is 4.76. Add 0.005 mol of HCl. The 0.005 mol of H⁺ consumes 0.005 mol of A⁻ and produces 0.005 mol of HA. The amounts for the buffer calculation are therefore 0.015 mol of A⁻ and 0.025 mol of HA.
The ratio of base to acid is 0.015 ÷ 0.025 = 0.60. The pH is approximately 4.76 + log₁₀(0.60) = 4.54. Before the addition, the amounts were equal and the estimated pH was 4.76. The decrease agrees with the addition of acid.
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CHAPTER 3 OF 4 · Back to contents
Check how much base was available
If 0.020 mol of HCl is added to the same starting buffer, the stoichiometric calculation uses up the available A⁻. At this point calculate the pH from the weak acid and its equilibrium, using the final volume where needed. If more than 0.020 mol is added, account for the excess strong acid and use the data and assumptions in the question.
For a separate check, try adding 0.005 mol of strong base to the starting mixture. OH⁻ converts HA into A⁻. The final amounts are 0.015 mol HA and 0.025 mol A⁻, giving pH ≈ 4.98.
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CHAPTER 4 OF 4 · Back to contents
Frequently asked questions
Why is the conjugate base on top?
Rearrange Kₐ = [H⁺][A⁻]/[HA] for [H⁺] and take the negative logarithm. The resulting pH expression puts [A⁻] in the numerator of the logarithm.
Does adding water alone have the same effect as adding acid?
Water lowers both component concentrations by the same factor and leaves their ratio unchanged in the buffer approximation. Dilution still reduces the capacity of a fixed volume of solution to resist added acid or base.
What is a quick way to check the sign?
After acid is added to a buffer with equal starting amounts, the base-to-acid ratio falls below 1. Its logarithm is negative, so the resulting pH should be below pKₐ.
Sources and next reading
Read the official SEAB syllabus
A-Level 2027 H2 Mathematics 9758 | Why a p-Value Can Pass 5% but Not 1%