Acid-base equilibriaEdexcel A-Level Chemistry: Topic test

20 questions, 54 marks

Edexcel A-Level Chemistry

Acid-base equilibria topic test

Total 54 marks

Name

Class

Date

1
A technician analyses an aqueous solution of hydrogensulfate ions. In water, hydrogensulfate ions act as an acid: HSO₄⁻(aq) + H₂O(l) ⇌ SO₄²⁻(aq) + H₃O⁺(aq). A sample of the solution has a pH of 2.40 at 298 K.
(a)
Which pair of species is a conjugate acid–base pair in this reaction?
[1 mark]
AHSO₄⁻ and H₃O⁺
BSO₄²⁻ and H₃O⁺
CH₂O and SO₄²⁻
DHSO₄⁻ and SO₄²⁻
(b)
What is the hydrogen ion concentration of the sample?
[1 mark]
A2.4 mol dm⁻³
B3.98 × 10⁻³ mol dm⁻³
C2.51 × 10⁻¹² mol dm⁻³
D0.380 mol dm⁻³
(c)
Explain, in terms of proton transfer, why the reaction between hydrogensulfate ions and water is an acid–base reaction.
[2 marks]
Total for question 1: 4 marks
2
Benzoic acid, C₆H₅COOH, is a weak monobasic acid used as a food preservative. At 298 K its acid dissociation constant, Ka, is 6.30 × 10⁻⁵ mol dm⁻³. A student prepares a 0.0200 mol dm⁻³ solution of benzoic acid.
(a)
What is the pH of the 0.0200 mol dm⁻³ solution of benzoic acid?
[1 mark]
A2.95
B1.70
C5.90
D4.20
(b)
What is the pKa of benzoic acid at 298 K?
[1 mark]
A9.80
B5.20
C4.20
D−4.20
(c)
State two assumptions made when calculating the pH of the benzoic acid solution from its Ka.
[2 marks]
Total for question 2: 4 marks
3
At 298 K the ionic product of water, Kw, is 1.00 × 10⁻¹⁴ mol² dm⁻⁶. A student has three solutions, each of concentration 0.0800 mol dm⁻³: lithium hydroxide, a strong base; hydrobromic acid, HBr, a strong acid; and a weak monobasic acid, HX, which has a pH of 2.90.
(a)
Calculate the pH of the lithium hydroxide solution.
[3 marks]
(b)
Each of the solutions of hydrobromic acid and of HX is diluted 100 times with water. Calculate the pH of the diluted hydrobromic acid. Explain why the pH of the diluted solution of HX is about 3.90, an increase of 1.00, rather than 4.90.
[4 marks]
Total for question 3: 7 marks
4
A student titrates 25.0 cm³ of 0.150 mol dm⁻³ propanoic acid, a weak monobasic acid with Ka = 1.35 × 10⁻⁵ mol dm⁻³, with 0.150 mol dm⁻³ sodium hydroxide solution at 298 K. The pH of the mixture is 2.85 at the start, 4.87 after 12.5 cm³ of alkali have been added and 8.9 at the equivalence point. The steep section of the titration curve spans approximately pH 7 to pH 11. Two indicators are available: methyl orange (colour change over the range pH 3.1–4.4) and phenolphthalein (colour change over the range pH 8.3–10.0).
(a)
Explain the three pH values given for this titration, and use them to show how the value of Ka can be found from the titration curve.
[6 marks]
(b)
Evaluate which of the two indicators is suitable for this titration. Explain also why the enthalpy change of neutralisation of propanoic acid with sodium hydroxide is less exothermic than that of hydrochloric acid with sodium hydroxide.
[6 marks]
Total for question 4: 12 marks
5
Lactic acid, CH₃CH(OH)COOH (HA), is a weak monobasic acid with Ka = 1.38 × 10⁻⁴ mol dm⁻³ at 298 K. A buffer solution is made containing 0.200 mol dm⁻³ lactic acid and 0.100 mol dm⁻³ sodium lactate.
(a)
What is the pH of the buffer solution?
[1 mark]
A3.86
B4.16
C3.56
D2.28
(b)
A small amount of sodium hydroxide solution is added to the buffer. Which statement explains why the pH changes very little?
[1 mark]
AThe added OH⁻ ions react with lactic acid to form lactate ions, so the ratio [HA] ÷ [A⁻] changes only slightly
BThe added OH⁻ ions react with lactate ions to form more lactic acid
CThe added OH⁻ ions react with sodium ions, so none remain in solution
DThe lactic acid is completely used up, so no further change can occur
(c)
A small amount of dilute hydrochloric acid is added to the buffer. Explain why the pH changes only slightly.
[2 marks]
Total for question 5: 4 marks
6
25.0 cm³ of 0.100 mol dm⁻³ aqueous ammonia, a weak base, is titrated with 0.100 mol dm⁻³ hydrochloric acid, a strong acid. The steep section of the titration curve spans approximately pH 3 to pH 6.5. Three indicators are available: methyl orange (colour change over the range pH 3.1–4.4), bromothymol blue (pH 6.0–7.6) and phenolphthalein (pH 8.3–10.0).
(a)
Approximately what is the pH at the equivalence point of this titration?
[1 mark]
AAbout 1
BAbout 5
CAbout 7
DAbout 9
(b)
Which indicator is suitable for this titration, and why?
[1 mark]
APhenolphthalein, because it changes colour at a pH above 7
BBromothymol blue, because it changes colour near pH 7
CPhenolphthalein, because the ammonia solution is alkaline
DMethyl orange, because its colour change range lies within the steep section of the curve
(c)
Explain why the pH at the equivalence point of this titration is below 7.
[2 marks]
Total for question 6: 4 marks
7
A student dissolves 0.510 g of pentanoic acid, C₄H₉COOH (molar mass 102.0 g mol⁻¹), a weak monobasic acid, in water and makes the solution up to 250 cm³. The pH of the solution at 298 K is measured as 3.27.
(a)
Calculate the acid dissociation constant, Ka, of pentanoic acid, with units.
[3 marks]
(b)
State two assumptions made in the calculation of Ka from the pH, and calculate the pKa of pentanoic acid.
[4 marks]
Total for question 7: 7 marks
8
Blood plasma is kept at a pH of about 7.40 by a buffer system containing carbonic acid, H₂CO₃, and hydrogencarbonate ions, HCO₃⁻. In the laboratory, a technician wants to prepare a buffer of pH 5.00 from butanoic acid (Ka = 1.51 × 10⁻⁵ mol dm⁻³ at 298 K) and sodium butanoate, C₃H₇COONa (molar mass 110.0 g mol⁻¹). The solution is to be 0.250 mol dm⁻³ in butanoic acid.
(a)
Explain how the carbonic acid and hydrogencarbonate buffer in blood limits the change in pH when excess acid or excess alkali enters the blood.
[6 marks]
(b)
Calculate the concentration of sodium butanoate needed in the buffer, and the mass of sodium butanoate required to prepare 500 cm³ of the buffer.
[6 marks]
Total for question 8: 12 marks

End of questions

Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).