Acids, Bases and Buffers | OCR A-Level Chemistry A (H432)
Acids, Bases and Buffers
- 120 questions
- 9 subtopics
- Module 5: Physical chemistry and transition elements
- Component 01 and Component 03
Brønsted-Lowry acids and bases, Ka and pKa, calculating pH for strong and weak acids and for strong bases, how a buffer works and the one that controls blood pH, and the titration curves and the indicators that suit them..
It covers brønsted-Lowry acids and bases and their reactions, ka, pKa, pH and the ionic product of water, calculating the pH of strong acids, strong bases and weak acids, pH and hydrogen ion concentration, what a buffer is and how it resists a change in pH, buffer calculations and the buffer that controls blood pH, titration curves, choosing an indicator and using a pH meter, pH titration curves for the four acid-base combinations and how an indicator works and choosing the right one.
Sample questions from Acids, Bases and Buffers
Answer each one closed book first, then open the answer.
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Brønsted-Lowry acids and bases and their reactions
When a base accepts a proton, what is formed?
Show the answer
Its conjugate acid. -
Ka, pKa, pH and the ionic product of water
State the relationship between pKₐ and Kₐ.
Show the answer
pKₐ = −log Kₐ -
Calculating the pH of strong acids, strong bases and weak acids
State the formula used to calculate pH of a strong base using Kw.
Show the answer
pH = −log(Kw / [OH⁻]) -
pH and hydrogen ion concentration
State the formula for calculating hydrogen ion concentration from pH.
Show the answer
[H⁺] = 10⁻ᵖᴴ -
What a buffer is and how it resists a change in pH
Give a specific example of forming a buffer from an excess of weak acid reacting with a strong alkali.
Show the answer
Excess ethanoic acid reacting with sodium hydroxide. -
Buffer calculations and the buffer that controls blood pH
How can the hydrogen ion concentration in a buffer be calculated?
Show the answer
By rearranging the Kₐ expression: [H⁺] = Kₐ × [acid]/[conjugate base]. -
Titration curves, choosing an indicator and using a pH meter
What requirement must an indicator meet to be suitable for a particular titration?
Show the answer
Its pH range (colour change range) must fall within the steep vertical section of the titration curve. -
pH titration curves for the four acid-base combinations
What happens to the pH after the equivalence point in a strong acid–strong base titration?
Show the answer
The pH levels off at high values as excess base is added.
The 9 subtopics
One subtopic is one session. Work down the list.
| Subtopic | What it covers | Questions |
|---|---|---|
| Brønsted-Lowry acids and bases and their reactions | Recall questions on Brønsted–Lowry acids and bases, conjugate acid–base pairs, monobasic, dibasic and tribasic acids, and the ionic equations for acids reacting with metals, carbonates, bases and alkalis. | 14 |
| Ka, pKa, pH and the ionic product of water | Recall questions on the Ka expression, what a larger Ka and a smaller pKa mean, the definition of pH, and the ionic product of water and its value at 25°C. | 10 |
| Calculating the pH of strong acids, strong bases and weak acids | Recall questions on pH from a strong monobasic acid, using Kw to reach pH for a strong base, and the two approximations behind the weak acid calculation. | 16 |
| pH and hydrogen ion concentration | Recall questions on converting each way between pH and hydrogen ion concentration, and on why a logarithmic scale is used at all. | 8 |
| What a buffer is and how it resists a change in pH | Recall questions on the two ways a buffer is made, the role of the weak acid and of its conjugate base, and what happens when acid or alkali is added. | 13 |
| Buffer calculations and the buffer that controls blood pH | Recall questions on calculating hydrogen ion concentration and pH from Ka and the two concentrations, and on the carbonic acid-hydrogencarbonate system in blood. | 9 |
| Titration curves, choosing an indicator and using a pH meter | Recall questions on the features of each curve, what an indicator must satisfy, why no indicator suits a weak acid with a weak base, and calibrating a pH meter. | 13 |
| pH titration curves for the four acid-base combinations | Recall questions on the starting pH, the buffer region, the equivalence point and the final pH for each of the four combinations. | 17 |
| How an indicator works and choosing the right one | Recall questions on the indicator equilibrium, which form dominates in acid and in alkali, the colour change range, and matching an indicator to a titration. | 20 |
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 sections
OCR A-Level Chemistry A Active Recall Guide
Every section, not just this one. 1,913 questions with their answers.