Acid-Base Equilibria | Edexcel A-Level Chemistry (9CH0)
Acid-Base Equilibria
- 73 questions
- 5 subtopics
- Paper 1: advanced inorganic and physical chemistry
- Paper 1 and Paper 3
Covers the Brønsted-Lowry definitions and the pH scale, strong and weak acids with Ka and Kw, pH calculations and titration curves, and buffers including the ones that hold blood pH steady..
It covers bronsted-Lowry acids and bases, and the pH scale, strong and weak acids, Ka and Kw, calculating pH, and titration curves, buffer solutions and the Henderson-Hasselbalch equation and buffers in blood, and the core practical.
Sample questions from Acid-Base Equilibria
Answer each one closed book first, then open the answer.
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Bronsted-Lowry acids and bases, and the pH scale
What forms when an acid donates a proton?
Show the answer
The acid forms its conjugate base. -
Bronsted-Lowry acids and bases, and the pH scale
Once hydrogen ion concentration is known, how is pH calculated?
Show the answer
pH = −log₁₀[H⁺] -
Strong and weak acids, Ka and Kw
What assumption is made about the equilibrium concentration of a weak acid HA when the degree of dissociation is small?
Show the answer
The equilibrium concentration of HA approximately equals the initial concentration of the acid. -
Strong and weak acids, Ka and Kw
How is hydrogen ion concentration calculated from hydroxide ion concentration using Kw?
Show the answer
[H⁺] = Kw / [OH⁻] -
Calculating pH, and titration curves
What quantities are plotted on the axes of a titration curve?
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pH is plotted on the vertical axis and volume of titrant added is plotted on the horizontal axis. -
Calculating pH, and titration curves
Over what pH range does an indicator typically change colour?
Show the answer
Approximately pKₐ ± 1. -
Buffer solutions and the Henderson-Hasselbalch equation
What products are formed when hydroxide ions react with the weak acid in an acidic buffer solution?
Show the answer
Water and the conjugate base of the weak acid -
Buffer solutions and the Henderson-Hasselbalch equation
Why does pH equal pKₐ at the half-equivalence point?
Show the answer
Because [A⁻] = [HA], so log₁₀([A⁻]/[HA]) = 0, and the Henderson-Hasselbalch equation simplifies to pH = pKₐ.
The 5 subtopics
One subtopic is one session. Work down the list.
| Subtopic | What it covers | Questions |
|---|---|---|
| Bronsted-Lowry acids and bases, and the pH scale | Recall questions on proton transfer and conjugate pairs, on the definition of pH, and on converting between pH and hydrogen ion concentration. | 12 |
| Strong and weak acids, Ka and Kw | Recall questions on full and partial dissociation, on writing Ka and pKa, and on the ionic product of water and its role in alkali calculations. | 16 |
| Calculating pH, and titration curves | Recall questions on the pH of strong and weak acids and of alkalis, on the effect of dilution, and on the four titration curve shapes and their indicators. | 17 |
| Buffer solutions and the Henderson-Hasselbalch equation | Acidic and basic buffers and how an acidic buffer removes added H⁺ and OH⁻ ions, the Henderson–Hasselbalch equation, why pH equals pKₐ at half-equivalence so Kₐ can be read from a titration curve, and why a weak acid gives a less exothermic enthalpy of neutralisation. | 15 |
| Buffers in blood, and the core practical | How the carbonic acid and hydrogencarbonate buffer holds blood near pH 7.4, Core Practical 9 finding Kₐ from the pH of a weak acid of known concentration, ways of measuring pH, the role of indicators, and decimal places, calculator functions and logarithms in pH calculations. | 13 |
How the guide is worked
Answering a question from memory stores it far better than reading the answer again. The guide runs that as a fixed procedure on one subtopic at a time, about twenty minutes a session.
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Step 1 · Closed book
Cover the answers. Work through one subtopic and write down what you can. Leave blanks where you have nothing.
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Step 2 · Open book
Go back to the top. Read each printed answer and write it out in full, including the ones you had right.
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Step 3 · Closed book again
Same questions, same order, from memory. The gap between pass one and pass three is the session result.
Read the full method, the return schedule and the research behind it.
Nearby topics
Edexcel A-Level Chemistry Active Recall Guide
Every topic, not just this one. 1,534 questions with their answers.