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.
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⁻].
Last updated: March 2026 · 3 flashcards · 3 quiz questions