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O Level Chemistry Practice Paper 3

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Questions

TuitionGoWhere Practice Paper - Chemistry O-Level

TuitionGoWhere Exam Practice (AI)

Subject: Chemistry
Level: O-Level (6092)
Paper: Practice Paper (Version 3 of 5)
Topic: Acids, Bases and Salts
Duration: 1 Hour
Total Marks: 40

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 on the question paper.
The number of marks is given in brackets [ ] at the end of each question or part question.
A copy of the Periodic Table is printed on page 12 (not included in this extract, assume standard data).
You may use a calculator.

Section A: Structured Questions (30 Marks)

Answer all questions in this section.

1. The pH scale is used to measure the acidity or alkalinity of a solution. Universal Indicator is often used to estimate pH.

(a) State the colour of Universal Indicator in a solution with pH 1.
[1]
........................................................................................................................................

(b) Explain, in terms of ions, why an aqueous solution of ethanoic acid ( $CH_3COOH$ ) has a higher pH than an aqueous solution of hydrochloric acid ( $HCl$ ) of the same concentration.
[2]
........................................................................................................................................
........................................................................................................................................
........................................................................................................................................

(c) Write the ionic equation for the neutralisation reaction between any strong acid and any strong alkali.
[1]
........................................................................................................................................

2. Zinc oxide is an amphoteric oxide.

(a) Define the term amphoteric.
[1]
........................................................................................................................................

(b) Write balanced chemical equations, including state symbols, for the reactions of zinc oxide with:
(i) dilute sulfuric acid.
[2]
........................................................................................................................................

(ii) aqueous sodium hydroxide. (Note: The product is sodium zincate, $Na_2ZnO_2$ , and water).
[2]
........................................................................................................................................

3. A student is given two white solids, Solid A and Solid B. One is ammonium chloride and the other is sodium chloride.

(a) Describe a chemical test, including the expected observations, to distinguish between Solid A and Solid B using sodium hydroxide solution and heat.
[3]
Test: ...........................................................................................................................
........................................................................................................................................
Observation for Ammonium Chloride: ....................................................................
........................................................................................................................................
Observation for Sodium Chloride: ..........................................................................
........................................................................................................................................

(b) The student dissolves the ammonium chloride in water and adds dilute nitric acid followed by aqueous silver nitrate.
(i) State the observation.
[1]
........................................................................................................................................
(ii) Write the ionic equation for this reaction.
[1]
........................................................................................................................................

4. Salts can be prepared by various methods depending on their solubility and the reactivity of the starting materials.

(a) Name the method used to prepare each of the following salts:
(i) Potassium sulfate from aqueous potassium hydroxide and dilute sulfuric acid.
[1]
........................................................................................................................................
(ii) Lead(II) iodide from aqueous lead(II) nitrate and aqueous potassium iodide.
[1]
........................................................................................................................................
(iii) Zinc sulfate from excess zinc carbonate and dilute sulfuric acid.
[1]
........................................................................................................................................

(b) Describe the essential steps to obtain pure, dry crystals of zinc sulfate from the mixture in (a)(iii) after the reaction is complete.
[3]
........................................................................................................................................
........................................................................................................................................
........................................................................................................................................
........................................................................................................................................

5. Copper(II) sulfate crystals can be prepared by reacting copper(II) oxide with dilute sulfuric acid.

(a) Why is copper metal not suitable for preparing copper(II) sulfate using dilute sulfuric acid?
[1]
........................................................................................................................................

(b) In an experiment, 25.0 cm³ of 1.0 mol/dm³ sulfuric acid is reacted with excess copper(II) oxide.
(i) Calculate the number of moles of sulfuric acid used.
[2]
........................................................................................................................................
........................................................................................................................................

(ii) Calculate the maximum mass of copper(II) sulfate crystals ( $CuSO_4 \cdot 5H_2O$ ) that can be formed.
( $A_r$ : Cu = 64, S = 32, O = 16, H = 1)
[3]
........................................................................................................................................
........................................................................................................................................
........................................................................................................................................
........................................................................................................................................

6. The table below shows the results of testing three unknown solutions P, Q, and R.

Solution	Action of Heat with NaOH(aq)	Action of $Ba(NO_3)_2$ (aq) + $HNO_3$ (aq)	Action of $AgNO_3$ (aq) + $HNO_3$ (aq)
P	Gas evolved that turns damp red litmus blue	White precipitate	No change
Q	No change	No change	White precipitate
R	No change	White precipitate	No change

(a) Identify the cation present in solution P.
[1]
........................................................................................................................................

(b) Identify the anion present in solution Q.
[1]
........................................................................................................................................

(c) Identify the anion present in solution R.
[1]
........................................................................................................................................

(d) Solution P is ammonium sulfate. Write the chemical formula for ammonium sulfate.
[1]
........................................................................................................................................

Section B: Free Response Questions (10 Marks)

Answer all questions in this section.

7. Sulfuric acid is a strong diprotic acid. It ionises in two stages in water:

Stage 1: $H_2SO_4(aq) \rightarrow H^+(aq) + HSO_4^-(aq)$
Stage 2: $HSO_4^-(aq) \rightleftharpoons H^+(aq) + SO_4^{2-}(aq)$

(a) Explain why Stage 1 is represented by a single arrow ( $\rightarrow$ ) while Stage 2 is represented by a reversible arrow ( $\rightleftharpoons$ ).
[2]
........................................................................................................................................
........................................................................................................................................

(b) A solution of sulfuric acid has a concentration of 0.05 mol/dm³. Assuming complete ionisation for simplicity in this calculation, calculate the concentration of $H^+$ ions in the solution.
[2]
........................................................................................................................................
........................................................................................................................................

(c) Magnesium reacts with dilute sulfuric acid.
(i) Write the balanced chemical equation for this reaction.
[2]
........................................................................................................................................

(ii) Explain why the rate of reaction decreases as the reaction proceeds, referring to collision theory.
[2]
........................................................................................................................................
........................................................................................................................................

(d) Suggest one method, other than changing the concentration of the acid, to increase the initial rate of reaction between magnesium and sulfuric acid. Explain your answer in terms of particle collisions.
[2]
........................................................................................................................................
........................................................................................................................................

[END OF PAPER]

Answers

TuitionGoWhere Practice Paper - Chemistry O-Level (Answer Key)

Topic: Acids, Bases and Salts
Version: 3 of 5

Section A: Structured Questions

1.
(a) Red [1]
(b) Hydrochloric acid is a strong acid and ionises completely in water to produce a high concentration of $H^+$ ions. Ethanoic acid is a weak acid and only partially ionises in water, producing a lower concentration of $H^+$ ions. Since pH is inversely related to $[H^+]$ , the lower $[H^+]$ results in a higher pH. [2]
(1 mark for mentioning complete vs partial ionisation; 1 mark for linking to $H^+$ concentration/pH)
(c) $H^+(aq) + OH^-(aq) \rightarrow H_2O(l)$ [1]

2.
(a) An amphoteric substance is one that can react with both acids and bases. [1]
(b)
(i) $ZnO(s) + H_2SO_4(aq) \rightarrow ZnSO_4(aq) + H_2O(l)$ [2]
(1 mark for correct formulae, 1 mark for balancing and state symbols)
(ii) $ZnO(s) + 2NaOH(aq) \rightarrow Na_2ZnO_2(aq) + H_2O(l)$ [2]
(1 mark for correct formulae, 1 mark for balancing and state symbols)

3.
(a) Test: Add aqueous sodium hydroxide to the solid and warm/heat gently. [1]
Observation for Ammonium Chloride: A pungent gas is evolved that turns damp red litmus paper blue. [1]
Observation for Sodium Chloride: No observable change / No gas evolved. [1]
(b)
(i) White precipitate. [1]
(ii) $Ag^+(aq) + Cl^-(aq) \rightarrow AgCl(s)$ [1]

4.
(a)
(i) Titration [1]
(ii) Precipitation [1]
(iii) Filtration followed by crystallisation (or Reaction of acid with excess insoluble base/carbonate) [1]
(Accept "Excess solid method" or similar)
(b)

Filter the mixture to remove excess unreacted zinc carbonate. [1]
Heat the filtrate to evaporate some water until the saturation point is reached (or until crystals form on a glass rod). [1]
Allow the solution to cool to crystallise, then filter, wash with distilled water, and dry between filter papers. [1]

5.
(a) Copper is below hydrogen in the reactivity series and does not react with dilute acids. [1]
(b)
(i) Volume in $dm^3 = 25.0 / 1000 = 0.025 dm^3$ [1]
Moles = Concentration $\times$ Volume = $1.0 \times 0.025 = 0.025$ mol [1]
(ii) Equation: $CuO + H_2SO_4 \rightarrow CuSO_4 + H_2O$
Mole ratio $H_2SO_4 : CuSO_4 \cdot 5H_2O$ is 1:1.
Moles of crystals = 0.025 mol [1]
$M_r$ of $CuSO_4 \cdot 5H_2O = 64 + 32 + (4 \times 16) + 5(18) = 160 + 90 = 250$ [1]
Mass = Moles $\times M_r = 0.025 \times 250 = 6.25$ g [1]

6.
(a) Ammonium ( $NH_4^+$ ) [1]
(b) Chloride ( $Cl^-$ ) [1]
(Note: Q gives white ppt with AgNO3, no ppt with Ba(NO3)2. Cl- fits.)
(c) Sulfate ( $SO_4^{2-}$ ) [1]
(Note: R gives white ppt with Ba(NO3)2, no ppt with AgNO3. SO4 2- fits.)
(d) $(NH_4)_2SO_4$ [1]

Section B: Free Response Questions

7.
(a) Stage 1 involves a strong acid dissociation which goes to completion (100% ionisation), hence the single arrow. Stage 2 involves the weak acid ion $HSO_4^-$ which only partially dissociates and establishes an equilibrium, hence the reversible arrow. [2]
(b) Sulfuric acid is diprotic. If we assume complete ionisation for calculation:
$H_2SO_4 \rightarrow 2H^+ + SO_4^{2-}$
Concentration of $H^+ = 2 \times [H_2SO_4] = 2 \times 0.05 = 0.10$ mol/dm³. [2]
(1 mark for factor of 2, 1 mark for correct answer)
(c)
(i) $Mg(s) + H_2SO_4(aq) \rightarrow MgSO_4(aq) + H_2(g)$ [2]
(1 mark for correct formulae, 1 mark for balancing/states)
(ii) As the reaction proceeds, the concentration of $H^+$ ions decreases. This leads to fewer effective collisions per unit time between magnesium atoms and hydrogen ions, thus decreasing the rate of reaction. [2]
(1 mark for concentration decrease, 1 mark for collision frequency explanation)
(d) Method: Increase the temperature. [1]
Explanation: At higher temperatures, particles have more kinetic energy. This leads to more frequent collisions and, more importantly, a higher proportion of collisions having energy greater than or equal to the activation energy, increasing the rate. [1]
(Alternative: Use magnesium powder instead of ribbon. Explanation: Larger surface area leads to more frequent collisions.)