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O Level Chemistry Practice Paper 2

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TuitionGoWhere Practice Paper - Chemistry O-Level

TuitionGoWhere Exam Practice (AI)

Subject: Chemistry (6092)
Level: O-Level
Paper: Practice Paper 2 (Version 2 of 5)
Topic: Acids, Bases and Salts
Duration: 1 hour
Total Marks: 50

Name: ________________________
Class: ________________________
Date: ________________________

Instructions to Candidates:

Write your name, class, and date in the spaces above.
Answer all questions.
Write your answers in the spaces provided in this question paper.
The number of marks is given in brackets [ ] at the end of each question or part question.
You may use a calculator.
A copy of the Periodic Table is printed on page 12 (not included in this extract, assume standard data).

Section A: Structured Questions

Answer all questions in this section.

1. Hydrochloric acid is a strong acid, while ethanoic acid is a weak acid.
(a) Define the term weak acid.
...........................................................................................................................................................
........................................................................................................................................................... [1]

(b) Describe how you could distinguish between 0.1 mol/dm³ hydrochloric acid and 0.1 mol/dm³ ethanoic acid using:
(i) A pH meter.
...........................................................................................................................................................
........................................................................................................................................................... [1]

(ii) Magnesium ribbon.
...........................................................................................................................................................
........................................................................................................................................................... [2]

2. Copper(II) sulfate crystals can be prepared by reacting excess copper(II) carbonate with dilute sulfuric acid.
(a) Write a balanced chemical equation for this reaction, including state symbols.
........................................................................................................................................................... [2]

(b) Explain why excess copper(II) carbonate is used in this preparation.
...........................................................................................................................................................
........................................................................................................................................................... [1]

(c) Describe the steps required to obtain pure, dry crystals of copper(II) sulfate from the reaction mixture after the reaction has stopped.
...........................................................................................................................................................
...........................................................................................................................................................
...........................................................................................................................................................
........................................................................................................................................................... [3]

3. Ammonia is manufactured by the Haber Process.
$N_2(g) + 3H_2(g) \rightleftharpoons 2NH_3(g) \quad \Delta H = -92 \text{ kJ/mol}$
(a) State the catalyst used in the Haber Process.
........................................................................................................................................................... [1]

(b) Explain why a high pressure is used in the Haber Process, referring to both yield and rate.
...........................................................................................................................................................
...........................................................................................................................................................
........................................................................................................................................................... [2]

(c) Ammonia is a base. Write an ionic equation to show how ammonia reacts with water to form an alkaline solution.
........................................................................................................................................................... [1]

4. A student investigates the reaction between sodium hydroxide solution and dilute sulfuric acid using a pH meter.
(a) The initial pH of the sodium hydroxide solution is 13. As sulfuric acid is added, the pH decreases.
Sketch the shape of the pH curve for this titration on the axes below. Label the equivalence point.

pH
14 |
   |
   |
   |
   |
   |
   |
   |
   |
   |
 0 |________________________________________ Volume of H₂SO₄ added

[2]

(b) Name a suitable indicator for this titration and state the colour change at the end-point.
Indicator: ________________________
Colour change: ________________________ to ________________________ [2]

5. Zinc oxide is an amphoteric oxide.
(a) Define the term amphoteric.
........................................................................................................................................................... [1]

(b) Write balanced chemical equations for the reaction of zinc oxide with:
(i) Dilute hydrochloric acid.
........................................................................................................................................................... [1]

(ii) Aqueous sodium hydroxide.
........................................................................................................................................................... [1]

6. Barium chloride solution is added to a solution of sodium sulfate.
(a) Describe the observation.
........................................................................................................................................................... [1]

(b) Write the ionic equation for this reaction, including state symbols.
........................................................................................................................................................... [2]

(c) Why is dilute nitric acid added before testing for sulfate ions with barium nitrate?
...........................................................................................................................................................
........................................................................................................................................................... [1]

7. Calcium hydroxide is used in agriculture.
(a) State one use of calcium hydroxide in agriculture and explain why it is used.
Use: ________________________
Explanation: ________________________
........................................................................................................................................................... [2]

(b) Calcium hydroxide reacts with ammonium chloride when heated.
(i) Write the balanced chemical equation for this reaction.
........................................................................................................................................................... [2]

(ii) Describe a test for the gas produced, including the observation.
Test: ________________________
Observation: ________________________ [2]

8. The table below shows the results of tests carried out on an unknown salt, X.

Test	Observation
Add aqueous sodium hydroxide to solution of X.	Green precipitate formed, insoluble in excess.
Add dilute nitric acid followed by aqueous silver nitrate to solution of X.	White precipitate formed.

(a) Identify the cation present in salt X.
........................................................................................................................................................... [1]

(b) Identify the anion present in salt X.
........................................................................................................................................................... [1]

(c) Name salt X.
........................................................................................................................................................... [1]

(d) Write the ionic equation for the formation of the green precipitate.
........................................................................................................................................................... [1]

9. Magnesium reacts with dilute hydrochloric acid.
$Mg(s) + 2HCl(aq) \rightarrow MgCl_2(aq) + H_2(g)$
In an experiment, 0.12 g of magnesium was added to excess dilute hydrochloric acid.
(Relative atomic mass: Mg = 24. Molar volume of gas at r.t.p. = 24 dm³/mol)

(a) Calculate the number of moles of magnesium used.
...........................................................................................................................................................
........................................................................................................................................................... [1]

(b) Calculate the volume of hydrogen gas produced at r.t.p.
...........................................................................................................................................................
...........................................................................................................................................................
........................................................................................................................................................... [2]

10. Potassium nitrate is a soluble salt.
(a) Suggest a suitable method for preparing potassium nitrate from potassium hydroxide and nitric acid.
........................................................................................................................................................... [1]

(b) Why is this method not suitable for preparing copper(II) nitrate from copper(II) oxide and nitric acid?
...........................................................................................................................................................
........................................................................................................................................................... [1]

Section B: Free Response Questions

Answer all questions in this section.

11. Sulfuric acid is a strong diprotic acid.
(a) Explain what is meant by the term diprotic.
........................................................................................................................................................... [1]

(b) Sulfuric acid reacts with sodium carbonate.
(i) Write the balanced chemical equation for this reaction.
........................................................................................................................................................... [2]

(ii) Calculate the mass of sodium carbonate required to neutralise 25.0 cm³ of 0.50 mol/dm³ sulfuric acid.
(Relative formula mass: $Na_2CO_3 = 106$ )
...........................................................................................................................................................
...........................................................................................................................................................
...........................................................................................................................................................
...........................................................................................................................................................
........................................................................................................................................................... [3]

12. Iron(III) chloride can be prepared by passing chlorine gas over heated iron filings.
(a) Why is this method used instead of reacting iron with hydrochloric acid?
...........................................................................................................................................................
........................................................................................................................................................... [2]

(b) Iron(III) chloride is hygroscopic. Explain what this means and why it affects the storage of the salt.
...........................................................................................................................................................
...........................................................................................................................................................
........................................................................................................................................................... [2]

(c) When aqueous sodium hydroxide is added to a solution of iron(III) chloride, a reddish-brown precipitate is formed.
(i) Identify the precipitate.
........................................................................................................................................................... [1]

(ii) Write the ionic equation for this reaction.
........................................................................................................................................................... [1]

13. A student is given three white solids: sodium chloride, sodium carbonate, and sodium sulfate.
Describe a series of chemical tests that would allow the student to identify each solid. Include reagents, observations, and conclusions.
...........................................................................................................................................................
...........................................................................................................................................................
...........................................................................................................................................................
...........................................................................................................................................................
...........................................................................................................................................................
...........................................................................................................................................................
...........................................................................................................................................................
...........................................................................................................................................................
...........................................................................................................................................................
........................................................................................................................................................... [6]

14. Nitric acid is a strong oxidising agent.
(a) When copper reacts with concentrated nitric acid, nitrogen dioxide gas is produced.
(i) State the colour of nitrogen dioxide gas.
........................................................................................................................................................... [1]

(ii) Write the balanced chemical equation for the reaction between copper and concentrated nitric acid.
........................................................................................................................................................... [2]

(b) Dilute nitric acid reacts with magnesium to produce hydrogen gas, unlike its reaction with copper.
Explain this difference in reactivity.
...........................................................................................................................................................
...........................................................................................................................................................
........................................................................................................................................................... [2]

15. Lead(II) iodide is an insoluble salt.
(a) Name two suitable solutions that can be mixed to prepare a precipitate of lead(II) iodide.

________________________ [2]

(b) Write the ionic equation for the formation of lead(II) iodide.
........................................................................................................................................................... [1]

(c) Describe how you would obtain a pure, dry sample of lead(II) iodide from the reaction mixture.
...........................................................................................................................................................
...........................................................................................................................................................
...........................................................................................................................................................
........................................................................................................................................................... [3]

End of Paper

Answers

TuitionGoWhere Practice Paper - Chemistry O-Level

Answer Key & Marking Scheme
Paper: Practice Paper 2 (Version 2 of 5)
Topic: Acids, Bases and Salts

Section A: Structured Questions

1.
(a) A weak acid is an acid that only partially dissociates (or ionises) in water. [1]
(b)
(i) Hydrochloric acid will have a lower pH (around 1) than ethanoic acid (around 3) because it has a higher concentration of $H^+$ ions. [1]
(ii) Magnesium reacts more vigorously (faster rate of effervescence) with hydrochloric acid than with ethanoic acid. [1]
The concentration of $H^+$ ions is higher in hydrochloric acid, leading to more frequent successful collisions. [1]

2.
(a) $CuCO_3(s) + H_2SO_4(aq) \rightarrow CuSO_4(aq) + H_2O(l) + CO_2(g)$
[1 for correct formulae, 1 for balancing and state symbols]
(b) To ensure all the sulfuric acid reacts / to ensure the salt formed is not contaminated with acid. [1]
(c)

Filter the mixture to remove excess copper(II) carbonate. [1]
Heat/Evaporate the filtrate to the point of crystallisation (or until saturated). [1]
Allow to cool for crystals to form, then filter and dry between filter papers/in a desiccator. [1]

3.
(a) Iron (Fe). [1]
(b) High pressure increases the yield because there are fewer moles of gas on the product side (2 moles vs 4 moles), shifting equilibrium to the right. [1]
High pressure also increases the rate of reaction by increasing the frequency of collisions. [1]
(c) $NH_3 + H_2O \rightleftharpoons NH_4^+ + OH^-$ [1]

4.
(a) Curve starts at high pH (13), decreases gradually, then drops sharply through pH 7, and levels off at low pH. Equivalence point marked at pH 7. [2]
(b) Indicator: Methyl Orange [1]
Colour change: Yellow to Red (or Orange) [1]
(Note: Phenolphthalein is also acceptable: Pink to Colourless)

5.
(a) An amphoteric oxide is one that reacts with both acids and bases to form a salt and water. [1]
(b)
(i) $ZnO + 2HCl \rightarrow ZnCl_2 + H_2O$ [1]
(ii) $ZnO + 2NaOH \rightarrow Na_2ZnO_2 + H_2O$ (Sodium zincate) [1]

6.
(a) White precipitate formed. [1]
(b) $Ba^{2+}(aq) + SO_4^{2-}(aq) \rightarrow BaSO_4(s)$ [2]
(c) To remove carbonate ions (or other interfering ions) which also form white precipitates with barium ions. [1]

7.
(a) Use: To neutralise acidic soil. [1]
Explanation: Calcium hydroxide is a base/alkali and reacts with acids in the soil to raise the pH. [1]
(b)
(i) $Ca(OH)_2 + 2NH_4Cl \rightarrow CaCl_2 + 2H_2O + 2NH_3$ [2]
(ii) Test: Damp red litmus paper. [1]
Observation: Turns blue. [1]

8.
(a) Iron(II) / $Fe^{2+}$ [1]
(b) Chloride / $Cl^-$ [1]
(c) Iron(II) chloride [1]
(d) $Fe^{2+}(aq) + 2OH^-(aq) \rightarrow Fe(OH)_2(s)$ [1]

9.
(a) Moles of Mg = $0.12 / 24 = 0.005$ mol [1]
(b) From equation, 1 mol Mg produces 1 mol $H_2$ .
Moles of $H_2$ = 0.005 mol.
Volume = $0.005 \times 24 = 0.12$ dm³ (or 120 cm³). [2]

10.
(a) Titration. [1]
(b) Copper(II) oxide is insoluble in water, so the endpoint cannot be detected using an indicator in the same way (or excess solid can be filtered off, making titration unnecessary/unsuitable for precise neutralisation detection without indicator change visibility). Accept: Titration is for soluble base + acid; CuO is insoluble. [1]

Section B: Free Response Questions

11.
(a) Diprotic means one molecule of the acid can release two protons ( $H^+$ ions). [1]
(b)
(i) $Na_2CO_3 + H_2SO_4 \rightarrow Na_2SO_4 + H_2O + CO_2$ [2]
(ii)
Moles of $H_2SO_4 = 0.50 \times (25.0/1000) = 0.0125$ mol. [1]
Ratio $Na_2CO_3 : H_2SO_4$ is 1:1.
Moles of $Na_2CO_3 = 0.0125$ mol. [1]
Mass = $0.0125 \times 106 = 1.325$ g. [1]

12.
(a) Reacting iron with hydrochloric acid produces Iron(II) chloride ( $FeCl_2$ ), not Iron(III) chloride ( $FeCl_3$ ). [1]
Direct combination with chlorine ensures the iron is oxidised to the +3 state. [1]
(b) Hygroscopic means the substance absorbs water vapour from the air. [1]
It must be stored in a dry/airtight container to prevent it from dissolving in the absorbed water (deliquescence) or becoming impure. [1]
(c)
(i) Iron(III) hydroxide / $Fe(OH)_3$ [1]
(ii) $Fe^{3+}(aq) + 3OH^-(aq) \rightarrow Fe(OH)_3(s)$ [1]

13.
Step 1: Add dilute nitric acid to each solid.

The solid that effervesces (produces gas turning limewater milky) is sodium carbonate. [2]
The other two show no visible change.

Step 2: To the remaining two solutions (or solids dissolved in water), add dilute nitric acid followed by aqueous silver nitrate.

The one forming a white precipitate is sodium chloride. [2]
The one with no precipitate (or remaining clear) is sodium sulfate. (Note: Silver sulfate is slightly soluble, but usually no ppt forms in dilute conditions compared to AgCl. Alternatively, use Barium Nitrate for sulfate confirmation).

Alternative for Step 2: Add dilute nitric acid followed by barium nitrate to the remaining two.

The one forming a white precipitate is sodium sulfate. [2]
The one with no change is sodium chloride.

(Award marks for logical sequence, correct reagents, and distinct observations for all three). [6]

14.
(a)
(i) Brown / Reddish-brown. [1]
(ii) $Cu + 4HNO_3 \rightarrow Cu(NO_3)_2 + 2NO_2 + 2H_2O$ [2]
(b) Magnesium is above hydrogen in the reactivity series, so it can displace hydrogen from dilute acids. [1]
Copper is below hydrogen in the reactivity series, so it cannot displace hydrogen from dilute acids. [1]

15.
(a) Lead(II) nitrate solution AND Potassium iodide solution (or Sodium iodide). [2]
(b) $Pb^{2+}(aq) + 2I^-(aq) \rightarrow PbI_2(s)$ [1]
(c)

Filter the mixture to collect the precipitate. [1]
Wash the residue with distilled water to remove soluble impurities. [1]
Dry the residue in an oven or between filter papers. [1]

End of Marking Scheme