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Secondary 4 Combined Science Chemistry Preliminary Examination Paper 1

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Questions

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TuitionGoWhere Exam Practice (AI)

Secondary 4 Combined Science Chemistry - Preliminary Examination

Subject: Combined Science Chemistry
Level: Secondary 4
Paper: Preliminary Paper 1 (Version 1)
Duration: 1 hour 15 minutes
Total Marks: 65
Name: ____________________ Class: __________ Date: __________

Instructions to Candidates:

  1. Answer all questions.
  2. Write your answers in the spaces provided.
  3. For calculations, show all working clearly.
  4. Use the relative atomic masses provided: H=1, C=12, N=14, O=16, Na=23, Mg=24, Al=27, S=32, Cl=35.5, K=39, Ca=40.

Section A: Short Answer and Structured Questions (30 Marks)

Question 1 (a) State the pH value of a solution that is strongly alkaline. [1]

(b) Describe the change in colour of universal indicator when it is added to a solution of dilute hydrochloric acid. [1]

Question 2 A student is tasked with preparing a sample of pure sodium chloride. (a) Name the most suitable method for preparing this salt. [1]

(b) State the reactants required for this method. [1]

(c) Explain why the salt must be heated until the solution is saturated before cooling. [2]

Question 3 The following chemical reactions are carried out in a laboratory: I. HCl(aq)+NaOH(aq)→NaCl(aq)+H2O(l)\text{HCl(aq)} + \text{NaOH(aq)} \rightarrow \text{NaCl(aq)} + \text{H}_2\text{O(l)} II. Zn(s)+2HCl(aq)→ZnCl2(aq)+H2(g)\text{Zn(s)} + 2\text{HCl(aq)} \rightarrow \text{ZnCl}_2\text{(aq)} + \text{H}_2\text{(g)} III. CaCO3(s)+2HCl(aq)→CaCl2(aq)+H2O(l)+CO2(g)\text{CaCO}_3\text{(s)} + 2\text{HCl(aq)} \rightarrow \text{CaCl}_2\text{(aq)} + \text{H}_2\text{O(l)} + \text{CO}_2\text{(g)}

(a) Classify reaction I using a term from the list: {Addition, Neutralisation, Substitution, Redox}. [1]

(b) Classify reaction II using a term from the list: {Addition, Neutralisation, Substitution, Redox}. [1]

(c) For reaction III, state the observation made when the gas produced is bubbled through limewater. [1]

Question 4 A solution of an unknown salt X is tested. When aqueous silver nitrate is added, a white precipitate is formed. When dilute nitric acid is added to the precipitate, it remains insoluble. (a) Identify the anion present in salt X. [1]

(b) If salt X is found to be soluble in water, suggest a possible formula for salt X. [1]

Question 5 (a) Define the term 'strong acid'. [2]

(b) Give one example of a weak acid. [1]

Question 6 A student wants to measure exactly 25.0 cm325.0\text{ cm}^3 of 0.1 mol/dm30.1\text{ mol/dm}^3 sodium hydroxide solution for a titration. (a) Which piece of apparatus is most suitable for measuring this volume accurately? [1]

(b) Explain your choice of apparatus in (a). [1]

Question 7 (a) Write a balanced chemical equation, including state symbols, for the reaction between magnesium ribbon and dilute sulfuric acid. [3]

(b) State the test for the gas evolved in part (a) and the expected result. [2]

Section B: Data Interpretation and Calculations (35 Marks)

Question 8 A student reacts 2.4 g2.4\text{ g} of magnesium ribbon with an excess of 1.0 mol/dm31.0\text{ mol/dm}^3 hydrochloric acid. Mg(s)+2HCl(aq)→MgCl2(aq)+H2(g)\text{Mg(s)} + 2\text{HCl(aq)} \rightarrow \text{MgCl}_2\text{(aq)} + \text{H}_2\text{(g)}

(a) Calculate the number of moles of magnesium used. [2]

(b) Calculate the volume of hydrogen gas produced at room temperature and pressure (RTP). (Molar volume of gas at RTP = 24 dm3mol−124\text{ dm}^3\text{mol}^{-1}) [3]

(c) Calculate the mass of MgCl2\text{MgCl}_2 produced. [3]

Question 9 The table below shows the pH values of four different solutions.

SolutionpH Value
A2
B7
C9
D13

(a) Which solution is the strongest base? Explain your answer. [2]

(b) If solution A is dilute nitric acid, describe the change in pH if the solution is diluted with distilled water. Explain your reasoning. [3]

Question 10 A sample of an organic acid is suspected to be unsaturated. (a) Describe a chemical test to confirm if the organic acid is unsaturated. [2]

(b) State the observation if the acid is indeed unsaturated. [1]

Question 11 A student performs a titration to find the concentration of a solution of sulfuric acid (H2SO4\text{H}_2\text{SO}_4). 25.0 cm325.0\text{ cm}^3 of the acid is neutralized by 20.0 cm320.0\text{ cm}^3 of 0.5 mol/dm30.5\text{ mol/dm}^3 sodium hydroxide (NaOH\text{NaOH}). H2SO4(aq)+2NaOH(aq)→Na2SO4(aq)+2H2O(l)\text{H}_2\text{SO}_4\text{(aq)} + 2\text{NaOH(aq)} \rightarrow \text{Na}_2\text{SO}_4\text{(aq)} + 2\text{H}_2\text{O(l)}

(a) Calculate the number of moles of NaOH\text{NaOH} used. [2]

(b) Determine the number of moles of H2SO4\text{H}_2\text{SO}_4 that reacted. [2]

(c) Calculate the concentration of the sulfuric acid in mol/dm3\text{mol/dm}^3. [3]

Question 12 Explain, using the concept of proton transfer, why an acid reacts with a base to form a salt and water. [4]

Question 13 (a) State the chemical formula of the salt formed when potassium hydroxide reacts with phosphoric acid. [1]

(b) Describe how you would obtain a pure, dry sample of this salt from the resulting solution. [4]

Answers

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Answer Key - Combined Science Chemistry Preliminary Paper 1 (Version 1)

Section A

Question 1 (a) pH 11 to 14 (Accept any value in this range). [1] (b) Changes from green/yellow to red. [1]

Question 2 (a) Titration. [1] (b) Sodium hydroxide (NaOH\text{NaOH}) and Hydrochloric acid (HCl\text{HCl}). [1] (c) To ensure maximum yield of crystals upon cooling / To allow crystals to form as the solubility decreases with temperature. [2]

Question 3 (a) Neutralisation. [1] (b) Redox (or Substitution). [1] (c) Limewater turns milky/cloudy. [1]

Question 4 (a) Chloride (Cl−\text{Cl}^-). [1] (b) NaCl\text{NaCl} or KCl\text{KCl} or MgCl2\text{MgCl}_2 etc. [1]

Question 5 (a) An acid that completely ionizes/dissociates in aqueous solution to produce a high concentration of H+\text{H}^+ ions. [2] (b) Ethanoic acid / Citric acid. [1]

Question 6 (a) Pipette. [1] (b) It is designed to deliver a single, fixed volume with very high precision/accuracy. [1]

Question 7 (a) Mg(s)+H2SO4(aq)→MgSO4(aq)+H2(g)\text{Mg(s)} + \text{H}_2\text{SO}_4\text{(aq)} \rightarrow \text{MgSO}_4\text{(aq)} + \text{H}_2\text{(g)} [3] - 1 mark for correct formulae. - 1 mark for balancing. - 1 mark for all correct state symbols. (b) Test: Use a lighted splint. Result: Gas burns with a 'pop' sound. [2]

Section B

Question 8 (a) Moles of Mg=mass/molar mass=2.4/24=0.1 mol\text{Moles of Mg} = \text{mass} / \text{molar mass} = 2.4 / 24 = 0.1\text{ mol}. [2] (b) Mole ratio Mg : H2=1:1\text{Mole ratio Mg : H}_2 = 1 : 1. Moles of H2=0.1 mol\text{Moles of H}_2 = 0.1\text{ mol}. Volume=0.1×24=2.4 dm3\text{Volume} = 0.1 \times 24 = 2.4\text{ dm}^3. [3] (c) Mole ratio Mg : MgCl2=1:1\text{Mole ratio Mg : MgCl}_2 = 1 : 1. Moles of MgCl2=0.1 mol\text{Moles of MgCl}_2 = 0.1\text{ mol}. Molar mass of MgCl2=24+(35.5×2)=95 g/mol\text{Molar mass of MgCl}_2 = 24 + (35.5 \times 2) = 95\text{ g/mol}. Mass=0.1×95=9.5 g\text{Mass} = 0.1 \times 95 = 9.5\text{ g}. [3]

Question 9 (a) Solution D. [1] It has the highest pH value (13), indicating the highest concentration of OH−\text{OH}^- ions. [1] (b) pH increases (becomes less acidic/moves towards 7). [1] Dilution decreases the concentration of H+\text{H}^+ ions per unit volume. [2]

Question 10 (a) Add bromine water to the organic acid. [2] (b) The orange/brown bromine water is decolorized. [1]

Question 11 (a) Moles of NaOH=(0.5×20.0)/1000=0.01 mol\text{Moles of NaOH} = (0.5 \times 20.0) / 1000 = 0.01\text{ mol}. [2] (b) Mole ratio H2SO4:NaOH=1:2\text{Mole ratio H}_2\text{SO}_4 : \text{NaOH} = 1 : 2. Moles of H2SO4=0.01/2=0.005 mol\text{Moles of H}_2\text{SO}_4 = 0.01 / 2 = 0.005\text{ mol}. [2] (c) Concentration=moles/volume (dm3)\text{Concentration} = \text{moles} / \text{volume (dm}^3\text{)} Conc=0.005/(25.0/1000)=0.005/0.025=0.2 mol/dm3\text{Conc} = 0.005 / (25.0 / 1000) = 0.005 / 0.025 = 0.2\text{ mol/dm}^3. [3]

Question 12 Acids are proton (H+\text{H}^+) donors. [1] Bases are proton (H+\text{H}^+) acceptors. [1] In a neutralisation reaction, the acid transfers a proton to the base. [1] This forms water (H2O\text{H}_2\text{O}) and a salt consisting of the remaining ions. [1]

Question 13 (a) K3PO4\text{K}_3\text{PO}_4. [1] (b)

  • Heat the solution in an evaporating dish to concentrate it/until saturated. [1]
  • Allow the solution to cool to crystallize the salt. [1]
  • Filter the crystals from the remaining solution. [1]
  • Wash crystals with a small amount of cold distilled water and dry between filter papers. [1]