OxfordAQA International A Level Chemistry
OxfordAQA International A Level Chemistry 3.1.10 Acids and Bases Exam Questions
Free OxfordAQA International A Level Chemistry revision for 3.1.10 Acids and Bases: what you need to know, common mistakes and exam-style questions with answers. Covers Brønsted-Lowry acids and bases, pH, Kw, Ka and pKa, pH curves, indicators and buffers.
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About OxfordAQA International A Level Chemistry 3.1.10 Acids and Bases
Acids and bases is a calculation-heavy A2 topic examined in Unit 3. You need to define Brønsted-Lowry acids and bases, calculate pH from hydrogen ion concentration, use the ionic product of water Kw to find the pH of strong bases, and use Ka and pKa to calculate the pH of weak acids. You also need to sketch and interpret pH curves for all combinations of strong and weak monoprotic acids and bases, choose indicators, and explain how acidic and basic buffers work and calculate the pH of acidic buffers. Required practical 7 investigates pH changes in titrations. Examiners regularly set multi-step pH calculations.
What you need to know
- Brønsted-Lowry theory An acid is a proton donor and a base is a proton acceptor. Acid-base reactions involve proton transfer.
- pH pH = -log10[H+] and [H+] = 10^-pH. For a strong monoprotic acid, [H+] equals the acid concentration.
- Kw Kw = [H+][OH-] = 1.00 x 10^-14 mol2 dm-6 at 298 K. Kw increases with temperature, so neutral pH falls below 7 at higher temperatures. For a strong base, [H+] = Kw / [OH-].
- Weak acids Ka = [H+][A-] / [HA]. Assuming [H+] = [A-] and that [HA] is unchanged, [H+] = square root of (Ka x [HA]). pKa = -log10 Ka.
- pH curves and indicators Learn the shapes of curves for strong acid-strong base, strong acid-weak base, weak acid-strong base and weak acid-weak base. The indicator must change colour within the vertical section, so phenolphthalein suits weak acid-strong base and methyl orange suits strong acid-weak base.
- Buffers Acidic buffers contain a weak acid and its salt; basic buffers contain a weak base and its salt, such as ammonia and ammonium chloride. For an acidic buffer [H+] = Ka x [HA] / [A-]. Buffers resist pH changes because the equilibrium shifts to remove added H+ or OH-.
Common mistakes in 3.1.10 Acids and Bases
- Using Ka for a strong acid. Strong acids are fully dissociated, so [H+] equals the acid concentration.
- Forgetting to account for excess reagent after partial neutralisation. Work out moles left over, then divide by the total volume.
- Saying water is acidic at 50 C because pH is below 7. It is still neutral because [H+] = [OH-].
- Saying a buffer keeps pH constant. It resists changes in pH when small amounts of acid or base are added.
Try three 3.1.10 Acids and Bases exam-style questions
1. Calculate the pH of 0.0500 mol dm-3 sodium hydroxide at 298 K. Kw = 1.00 x 10^-14 mol2 dm-6. (2 marks)
Show the mark scheme answer
[H+] = 1.00 x 10^-14 / 0.0500 = 2.00 x 10^-13 mol dm-3; pH = 12.70.
2. Calculate the pH of 0.100 mol dm-3 ethanoic acid. Ka = 1.74 x 10^-5 mol dm-3. (2 marks)
Show the mark scheme answer
[H+] = square root of (1.74 x 10^-5 x 0.100) = 1.32 x 10^-3 mol dm-3; pH = 2.88.
3. A buffer contains 0.100 mol dm-3 ethanoic acid and 0.200 mol dm-3 sodium ethanoate. Ka = 1.74 x 10^-5 mol dm-3. Calculate its pH. (2 marks)
Show the mark scheme answer
[H+] = 1.74 x 10^-5 x 0.100 / 0.200 = 8.70 x 10^-6 mol dm-3; pH = 5.06.
3.1.10 Acids and Bases FAQs
What does OxfordAQA International A Level Chemistry 3.1.10 Acids and Bases cover?
It covers Brønsted-Lowry acids and bases, pH, Kw, Ka and pKa, pH curves, indicators and buffers.
Which unit is 3.1.10 Acids and Bases examined in?
It is examined in Unit 3, an International A2 paper covering inorganic and physical chemistry. The Unit 5 practical and synoptic paper can also assess it.
Which pH calculations do I need?
You need the pH of strong acids, strong bases using Kw, weak acids using Ka, solutions after partial neutralisation, and acidic buffers. Practise finding moles of excess reagent and dividing by the total volume.
