HSC Chemistry — Module 6
pH Calculations — Flashcards & Quiz
pH calculations are a core quantitative skill in HSC Chemistry Module 6. You need to confidently use pH = -log[H⁺], calculate [H⁺] and [OH⁻] from pH and pOH, and understand Ka for weak acids. Titration calculations and buffer chemistry extend these fundamentals. Exam questions range from straightforward pH calculations to interpreting titration curves and explaining why weak acid-strong base titrations have equivalence points above pH 7. Practise converting between pH, pOH, and concentrations until it becomes automatic.
Key Points
- pH = −log₁₀[H⁺]; pOH = −log₁₀[OH⁻]; pH + pOH = 14 at 25°C.
- Strong acids (HCl, HNO₃, H₂SO₄) fully dissociate — [H⁺] equals the molar concentration directly.
- Weak acids partially dissociate — use Ka = [H⁺][A⁻]/[HA]; for dilute weak acids, [H⁺] ≈ √(Ka × C).
- Kw = [H⁺][OH⁻] = 1.0 × 10⁻¹⁴ at 25°C; Kw is temperature-dependent — pH of neutral water drops slightly above 25°C.
- Buffers resist pH change when small amounts of acid/base are added — calculate with the Henderson-Hasselbalch equation: pH = pKa + log([A⁻]/[HA]).
- Exam rounding: HSC markers expect pH to 2 decimal places and significant figures in [H⁺] matching the data given.
Common Mistakes to Avoid
- Using [H⁺] = molar concentration for weak acids — weak acids only partially dissociate, so use √(Ka × C) as an approximation.
- Forgetting log rules: pH = -log[H⁺], NOT pH = log[H⁺]. The negative sign matters.
- Mixing up Ka (acid dissociation constant) and Kw (water ion product): Kw = 1.0 × 10⁻¹⁴ at 25°C.
- Using pH + pOH = 14 at temperatures other than 25°C — Kw is temperature-dependent.
- Rounding too early in the calculation — keep extra significant figures until the final step, then round to match the data given.
Exam Strategy
HSC Module 6 pH calculations usually come as a 3-5 mark question: "calculate the pH of a solution of X." Always start by writing the dissociation equation, then ask: is this a strong or weak acid/base? For strong, [H⁺] = molar concentration directly. For weak, use Ka = [H⁺][A⁻]/[HA] with the √(Ka × C) approximation. Buffers need the Henderson-Hasselbalch equation. Show your working step by step — method marks are typically 60% of the score.
Sample Flashcards
Q1: How is pH calculated and what does the pH scale represent?
pH = -log₁₀[H⁺]. The scale runs from 0-14 (in aqueous solutions at 25°C). pH < 7 = acidic, pH = 7 = neutral, pH > 7 = basic. Each whole pH unit represents a 10-fold change in [H⁺].
Q2: What is the relationship between pH, pOH and Kw?
Kw = [H⁺][OH⁻] = 1.0 × 10⁻¹⁴ at 25°C. pH + pOH = 14 (at 25°C). pOH = -log₁₀[OH⁻]. If you know pH, you can find [H⁺], [OH⁻] and pOH.
Q3: Calculate the pH of a 0.05 M strong acid solution.
For a strong monoprotic acid, [H⁺] = concentration because it fully dissociates. pH = -log₁₀[H⁺] = -log₁₀(0.05) = -log₁₀(5 × 10⁻²) = 1.30.
Sample Quiz Questions
Q1: A solution with a pH of 3 is more acidic than a solution with a pH of 5.
Answer: TRUE
Lower pH = more acidic = higher [H⁺]. pH 3 has [H⁺] = 10⁻³ M, which is 100× greater than pH 5 ([H⁺] = 10⁻⁵ M).
Q2: pH + pOH = 14 at all temperatures.
Answer: FALSE
pH + pOH = 14 ONLY at 25°C. At other temperatures, Kw changes and the sum differs. At higher T, Kw > 10⁻¹⁴ so pH + pOH < 14.
Q3: The pH of pure water increases above 7 at temperatures higher than 25°C.
Answer: FALSE
At higher temperatures, Kw increases (self-ionisation is endothermic), so [H⁺] increases and pH DECREASES below 7. But water is still neutral because [H⁺] = [OH⁻].
Revision Tip
Calculations need procedural fluency — build a Revizi deck of 10+ pH problems covering strong/weak acids, buffers, and dilutions, then drill until each one takes under two minutes.
Related Concepts
Last updated: March 2026 · 3 flashcards · 3 quiz questions