Secondary 4 Combined Science Chemistry Preliminary Examination Paper 5

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Secondary 4 Combined Science Chemistry - Preliminary Examination

Subject: Combined Science Chemistry
Level: Secondary 4
Paper: Paper 3 (Chemistry) - Version 5
Duration: 1 hour 15 minutes
Total Marks: 65
Name: ___________________________ Class: ___________ Date: ___________

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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: H=1, C=12, N=14, O=16, Na=23, Mg=24, Al=27, S=32, Cl=35.5, K=39, Ca=40.

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

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

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

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(c) Explain why a solution of sodium chloride has a pH of 7. [2]

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Question 2 A student is tasked with preparing a pure sample of copper(II) sulfate crystals from copper(II) oxide and dilute sulfuric acid. (a) State the name of the process used to remove the excess copper(II) oxide from the reaction mixture. [1]

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(b) Explain why the copper(II) oxide must be added in excess. [1]

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(c) Describe how the student can obtain the crystals from the resulting solution. [2]

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Question 3 The following reactions are carried out in a laboratory: Reaction I: $\text{Mg(s)} + 2\text{HCl(aq)} \rightarrow \text{MgCl}_2\text{(aq)} + \text{H}_2\text{(g)}$ Reaction II: $\text{NaOH(aq)} + \text{HCl(aq)} \rightarrow \text{NaCl(aq)} + \text{H}_2\text{O(l)}$ Reaction III: $\text{CaCO}_3\text{(s)} + 2\text{HCl(aq)} \rightarrow \text{CaCl}_2\text{(aq)} + \text{H}_2\text{O(l)} + \text{CO}_2\text{(g)}$

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

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(b) In Reaction I, identify the gas evolved and describe a chemical test to confirm its identity. [2] Gas: ______________________________________________________________________ Test: ______________________________________________________________________

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

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Question 4 A solution of an unknown salt X is reacted with aqueous silver nitrate. A white precipitate is formed. (a) Identify the anion present in salt X. [1]

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

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(c) Describe a chemical test to distinguish between a solution of sodium chloride and a solution of sodium nitrate. [2]

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

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(b) Give one example of a weak acid and state its common use. [2] Example: __________________________________________________________________ Use: ______________________________________________________________________

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Section B: Data Interpretation and Calculations (35 Marks)

Question 6 A student performs a titration to determine the concentration of a solution of sodium hydroxide ($\text{NaOH}$). $25.0\text{ cm}^3$ of $\text{NaOH}$ is neutralized by $20.0\text{ cm}^3$ of $0.10\text{ mol/dm}^3$ sulfuric acid ($\text{H}_2\text{SO}_4$). The equation for the reaction is: $2\text{NaOH(aq)} + \text{H}_2\text{SO}_4\text{(aq)} \rightarrow \text{Na}_2\text{SO}_4\text{(aq)} + 2\text{H}_2\text{O(l)}$

(a) Calculate the number of moles of $\text{H}_2\text{SO}_4$ used in the titration. [2]

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(b) Using the stoichiometry of the equation, calculate the number of moles of $\text{NaOH}$ that reacted. [1]

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(c) Calculate the concentration of the $\text{NaOH}$ solution in $\text{mol/dm}^3$. [2]

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Question 7 A sample of an impure carbonate salt contains $1.20\text{ g}$ of $\text{Na}_2\text{CO}_3$. When this sample is reacted with excess dilute $\text{HCl}$, a certain volume of $\text{CO}_2$ gas is collected at room temperature and pressure (RTP). (a) Calculate the relative formula mass of $\text{Na}_2\text{CO}_3$. [1]

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(b) Calculate the number of moles of $\text{Na}_2\text{CO}_3$ present in the sample. [2]

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(c) Calculate the volume of $\text{CO}_2$ gas evolved at RTP. (Molar volume of gas at RTP = $24\text{ dm}^3\text{/mol}$) [3]

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Question 8 The table below shows the solubility of three salts in water at $20^\circ\text{C}$.

Salt	Solubility ($\text{g/100g water}$)
Potassium Nitrate	31.6
Lead(II) Sulfate	0.004
Silver Chloride	0.00019

(a) Which salt is considered insoluble? Explain your answer based on the data. [2]

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(b) Suggest a method to increase the solubility of Potassium Nitrate in water. [1]

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(c) Describe how you would prepare a pure sample of Lead(II) Sulfate. [3]

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Question 9 Ammonia is produced industrially via the Haber Process. (a) State the reactants used in the production of ammonia. [1]

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(b) State the catalyst used in this process. [1]

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(c) Ammonia is a weak base. Explain why its pH is not 14 even when concentrated. [2]

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Question 10 A student is given three colorless solutions: $\text{HCl}$, $\text{NaOH}$, and $\text{KNO}_3$. (a) Describe how the student can identify the $\text{NaOH}$ solution using only a piece of red litmus paper. [2]

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(b) Once $\text{NaOH}$ is identified, describe how the student can distinguish between $\text{HCl}$ and $\text{KNO}_3$ using the $\text{NaOH}$ solution. [3]

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Answer Key - Combined Science Chemistry Secondary 4 (Prelim V5)

Section A

Question 1 (a) 11 to 14 (Accept any value > 10). [1] (b) Red. [1] (c) Sodium chloride is a salt formed from a strong acid ($\text{HCl}$) and a strong base ($\text{NaOH}$). [1] The ions $\text{Na}^+$ and $\text{Cl}^-$ do not hydrolyze in water / do not affect the $\text{H}^+$ and $\text{OH}^-$ balance. [1]

Question 2 (a) Filtration. [1] (b) To ensure all the sulfuric acid is neutralized / reacted. [1] (c) Heat the solution to evaporate some water until the saturation point is reached [1], then allow it to cool and crystallize. [1]

Question 3 (a) Neutralisation. [1] (b) Hydrogen [1]. Test: Place a lighted splint at the mouth of the test tube; the splint will extinguish with a 'pop' sound. [1] (c) Limewater turns milky / cloudy. [1]

Question 4 (a) Chloride ($\text{Cl}^-$). [1] (b) $\text{NaCl}$ (or any soluble chloride like $\text{KCl}$). [1] (c) Add aqueous silver nitrate to both. [1] Sodium chloride will form a white precipitate, while sodium nitrate will show no visible change. [1]

Question 5 (a) An acid that completely ionizes/dissociates in aqueous solution [1] to produce a high concentration of hydrogen ions ($\text{H}^+$). [1] (b) Example: Ethanoic acid / Citric acid. [1] Use: Vinegar / Food preservative / Flavoring. [1]

Section B

Question 6 (a) $\text{Moles} = \text{Concentration} \times \text{Volume} = 0.10 \times (20/1000) = 0.002\text{ mol}$. [2] (b) Ratio $\text{H}_2\text{SO}_4 : \text{NaOH} = 1 : 2$. Moles $\text{NaOH} = 0.002 \times 2 = 0.004\text{ mol}$. [1] (c) $\text{Concentration} = \text{moles} / \text{volume} = 0.004 / (25/1000) = 0.16\text{ mol/dm}^3$. [2]

Question 7 (a) $23(2) + 12 + 16(3) = 46 + 12 + 48 = 106$. [1] (b) $\text{Moles} = 1.20 / 106 = 0.0113\text{ mol}$. [2] (c) Ratio $\text{Na}_2\text{CO}_3 : \text{CO}_2 = 1 : 1$. Moles $\text{CO}_2 = 0.0113\text{ mol}$. [1] $\text{Volume} = 0.0113 \times 24 = 0.271\text{ dm}^3$ (or $271\text{ cm}^3$). [2]

Question 8 (a) Silver Chloride [1]. Its solubility is extremely low ($0.00019\text{ g/100g}$), meaning it does not dissolve significantly in water. [1] (b) Increase the temperature of the water. [1] (c) Mix two soluble salts, e.g., Lead(II) nitrate and Sodium sulfate [1]. Filter the resulting precipitate [1], wash with distilled water and dry. [1]

Question 9 (a) Nitrogen ($\text{N}_2$) and Hydrogen ($\text{H}_2$). [1] (b) Iron / Iron oxide. [1] (c) Ammonia is a weak base [1], meaning it only partially ionizes in water, resulting in a lower concentration of $\text{OH}^-$ ions. [1]

Question 10 (a) Dip red litmus paper into the solutions. [1] The solution that turns the red litmus paper blue is $\text{NaOH}$. [1] (b) Add $\text{NaOH}$ to both remaining solutions. [1] The one that produces a neutralization reaction (detected by adding an indicator or temperature rise) is $\text{HCl}$. [1] $\text{KNO}_3$ will show no reaction with $\text{NaOH}$. [1]