Acid-base EquilibriaEdexcel International A Level Chemistry: Topic test

20 questions, 54 marks

Edexcel International A Level Chemistry

Acid-base Equilibria topic test

Total 54 marks

Name

Class

Date

1
A laboratory cleans glassware with 0.0250 mol dm⁻³ hydrobromic acid, HBr, a strong acid. All measurements are at 298 K.
(a)
What is the pH of the hydrobromic acid?
[1 mark]
A2.60
B1.30
C1.60
D12.40
(b)
Why does 0.0250 mol dm⁻³ hydrobromic acid have a lower pH than 0.0250 mol dm⁻³ ethanoic acid?
[1 mark]
AHBr is almost completely dissociated in water, so it gives a higher [H⁺]
BHBr molecules contain more hydrogen atoms than ethanoic acid molecules.
CEthanoic acid is a Brønsted–Lowry base, so it removes H⁺ from the solution.
DEthanoic acid dissociates completely but HBr does not.
(c)
A 25.0 cm³ portion of the hydrobromic acid is mixed with 25.0 cm³ of pure water. Calculate the pH of the diluted solution.
[2 marks]
Total for question 1: 4 marks
2
The ionic product of water, $K_w$ , is 1.00 × 10⁻¹⁴ mol² dm⁻⁶ at 298 K and 2.92 × 10⁻¹⁴ mol² dm⁻⁶ at 313 K. A technician has a 0.0400 mol dm⁻³ solution of sodium hydroxide.
(a)
Which statement about pure water at 313 K is correct?
[1 mark]
AIts pH is 7.00, because $K_w$ does not change with temperature.
BIts pH is 6.77, so it is acidic at this temperature.
CIts pH is 7.23, so it is alkaline at this temperature.
DIts pH is 6.77 and it is neutral, because [H⁺] = [OH⁻]
(b)
What does the increase in $K_w$ between 298 K and 313 K show about the ionisation of water?
[1 mark]
AThe ionisation of water is exothermic.
BThe ionisation of water is endothermic
CWater becomes an acid at 313 K.
DThe concentration of OH⁻ is greater than that of H⁺ at 313 K.
(c)
Calculate the pH of the 0.0400 mol dm⁻³ sodium hydroxide solution at 313 K.
[2 marks]
Total for question 2: 4 marks
3
Propanoic acid, CH₃CH₂COOH, is a weak monoprotic acid with $K_a$ = 1.35 × 10⁻⁵ mol dm⁻³ at 298 K. A solution of the acid has a concentration of 0.250 mol dm⁻³.
(a)
Calculate the pH of the solution.
[3 marks]
(b)
State the two assumptions made in your calculation in (a). Calculate the percentage of the acid molecules that are dissociated and state what this shows about the assumption on the concentration of acid.
[4 marks]
Total for question 3: 7 marks
4
A student titrates 50.0 cm³ of 0.200 mol dm⁻³ methanoic acid, HCOOH, with 0.100 mol dm⁻³ sodium hydroxide solution at 298 K. For methanoic acid $K_a$ = 1.78 × 10⁻⁴ mol dm⁻³. Phenolphthalein changes colour over the pH range 8.3 to 10.0 and methyl orange over the range 3.1 to 4.4.
(a)
Calculate the pH of the mixture after 25.0 cm³ of the sodium hydroxide solution has been added.
[6 marks]
(b)
The titration is continued to the equivalence point. State the total volume of sodium hydroxide solution needed, explain why the pH at the equivalence point is above 7, and choose a suitable indicator, justifying your choice. Explain why the mixture in (a) resists a change in pH when a small amount of acid is added.
[6 marks]
Total for question 4: 12 marks
5
A student titrates 25.0 cm³ of 0.100 mol dm⁻³ ammonia solution with 0.100 mol dm⁻³ hydrochloric acid at 298 K. The ammonium ion has $K_a$ = 5.62 × 10⁻¹⁰ mol dm⁻³. The steep section of the titration curve spans about pH 6.5 to pH 4.0. Indicator ranges: phenolphthalein 8.3–10.0, bromothymol blue 6.0–7.6, methyl red 4.2–6.3, thymol blue (alkaline range) 8.0–9.6.
(a)
Which statement describes how the pH changes during this titration?
[1 mark]
AIt starts below 7, rises sharply near 25.0 cm³ of acid, and finishes above 12.
BIt starts above 7, falls sharply near 25.0 cm³ of acid, and finishes below 2
CIt starts at about 13, stays almost constant, then falls to 7 at equivalence.
DIt starts at 7 and falls gradually to below 2 with no sharp change.
(b)
Which indicator is most suitable for this titration?
[1 mark]
APhenolphthalein
BBromothymol blue
CMethyl red
DThymol blue (alkaline range)
(c)
Calculate the pH of the solution at the equivalence point.
[2 marks]
Total for question 5: 4 marks
6
Fermenting vegetables become more acidic as bacteria release lactic acid, CH₃CH(OH)COOH (pKa = 3.86 at 298 K). A food scientist adds sodium lactate to the brine, so that it contains both lactic acid and lactate ions.
(a)
Which row correctly identifies what removes added H⁺ ions and what removes added OH⁻ ions in this buffer?
[1 mark]
AAdded H⁺ is removed by lactic acid; added OH⁻ is removed by lactate ions.
BBoth are removed by lactate ions.
CBoth are removed by lactic acid.
DAdded H⁺ is removed by lactate ions; added OH⁻ is removed by lactic acid
(b)
What ratio [lactate] / [lactic acid] gives a pH of 4.86?
[1 mark]
A10
B0.10
C1.0
D4.86
(c)
Explain how the brine resists a change in pH when the bacteria release more lactic acid.
[2 marks]
Total for question 6: 4 marks
7
A 0.0500 mol dm⁻³ solution of a weak monoprotic acid HQ has a pH of 3.00 at 298 K.
(a)
Calculate the acid dissociation constant, $K_a$ , of HQ, including its units, and state the assumptions you make.
[3 marks]
(b)
The solution is diluted by a factor of 10. Calculate its new pH. Compare the change in pH with that for a strong acid solution that also has a pH of 3.00 before being diluted by a factor of 10, and explain the difference.
[4 marks]
Total for question 7: 7 marks
8
A student titrates 25.0 cm³ of 0.0500 mol dm⁻³ sodium carbonate solution with 0.100 mol dm⁻³ hydrochloric acid. The carbonate ion is a dibasic base: CO₃²⁻ + H⁺ → HCO₃⁻, then HCO₃⁻ + H⁺ → H₂CO₃. For carbonic acid, pKa₁ = 6.35 and pKa₂ = 10.33 at 298 K. Phenolphthalein changes colour over the range pH 8.3 to 10.0 and methyl orange over the range pH 3.1 to 4.4.
(a)
Calculate the volumes of acid at the first and second equivalence points. Explain which of the two indicators is suitable for detecting each equivalence point.
[6 marks]
(b)
Calculate the pH of the mixture after 6.25 cm³ of acid has been added and after 18.75 cm³ of acid has been added. Explain why the mixture after 6.25 cm³ resists a change in pH when a small amount of acid is added.
[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).