📋 Structured Questions
These questions test key calculation skills from Topic 3. Attempt each question on paper, then click "Show Mark Scheme" to check your answer.
Question 1: Percentage Yield
2 marksFor a Stage 2 reaction the percentage yield was 92.3%. The theoretical maximum mass of titanium produced in this batch was 13.5 kg.
(a) Calculate the actual mass of titanium produced. [2]
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- (actual mass =) 92.3/100 × 13.5 OR 0.923 × 13.5 [1]
- = 12.5 (kg) [1]
Question 2: Conservation of Mass
2 marksA student investigated the reaction between silver nitrate and sodium iodide. Mass before mixing: Beaker A = 78.26 g, Beaker B = 78.50 g. Mass after mixing: Beaker A = 108.22 g, Beaker B = 48.54 g.
(a) Explain how the results demonstrate the law of conservation of mass. Use data from the table. [2]
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- Total mass before = 156.76 g AND total mass after = 156.76 g [1]
- The mass of products equals the mass of reactants / no change in mass during the reaction / no atoms were lost or created [1]
Question 3: Atom Economy
3 marksNiO + C → Ni + CO. Relative atomic masses (Aᵣ): C = 12, Ni = 59. Relative formula mass (Mᵣ): NiO = 75.
(a) Calculate the percentage atom economy for the reaction to produce nickel. Give your answer to 3 significant figures. [3]
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- Total Mᵣ of reactants = 75 + 12 = 87 [1]
- % atom economy = (59 / 87) × 100 [1]
- = 67.8 (%) [1]
Question 4: Concentration
3 marksA student used a solution of sodium hydroxide of concentration 4.00 g/dm³. (1 dm³ = 1000 cm³)
(a) Calculate the mass of sodium hydroxide in 25.0 cm³ of this solution. [3]
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- 25.0 cm³ = 25.0 / 1000 = 0.025 dm³ [1]
- mass = 0.025 × 4.00 [1]
- = 0.1 (g) [1]
Question 5: Moles & Molar Gas Volume
3 marksThe volume of chlorine collected is 6.6 cm³. The volume of one mole of any gas at RTP is 24.0 dm³.
(a) Calculate the amount in moles of chlorine gas collected. Give your answer in standard form. [3]
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- Volume = 6.6 / 1000 = 0.0066 dm³ [1]
- Moles = 0.0066 / 24 [1]
- = 2.75 × 10⁻⁴ mol [1]
Question 6: Percentage by Mass
3 marksChlorine reacts with cyclohexene to produce a compound with the formula C₆H₁₀Cl₂. Relative atomic masses (Aᵣ): H = 1, C = 12, Cl = 35.5.
(a) Calculate the percentage by mass of chlorine in a molecule of C₆H₁₀Cl₂. [3]
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- Mᵣ of C₆H₁₀Cl₂ = (6 × 12) + (10 × 1) + (2 × 35.5) = 153 [1]
- % chlorine = (71 / 153) × 100 [1]
- = 46.4 (%) [1]
Question 7: Conservation of Mass ⭐ Extended Response
6 marksA student heats 10.0 g of calcium carbonate (CaCO₃) in a non-enclosed boiling tube. After the thermal decomposition is complete, the final mass of the solid residue (calcium oxide, CaO) is found to be significantly less than 10.0 g.
(a) Explain this apparent decrease in mass in terms of the law of conservation of mass and the particle model. [6]
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Level 3 (5–6 marks): A comprehensive explanation that defines the law of conservation of mass and logically explains the apparent mass "loss" by identifying the gaseous product and the open system.
Level 2 (3–4 marks): Identifies that a gas is produced and escapes, and partially links this to conservation of mass.
Level 1 (1–2 marks): Mentions conservation of mass or states that a gas is produced, but lacks a connected explanation.
Indicative content:
- The law of conservation of mass states that no atoms are lost or made during a chemical reaction — total mass of reactants equals total mass of products
- CaCO₃ → CaO + CO₂ — carbon dioxide gas is produced
- The boiling tube is non-enclosed, so the CO₂ gas escapes into the atmosphere
- The balance only measures the solid residue remaining, so the mass appears to decrease
- The total mass of the system (solid + gas) is conserved
Question 8: Required Practical — Titration 🔬
4 marksA student carries out a titration to find the volume of hydrochloric acid needed to neutralise 25.0 cm³ of sodium hydroxide solution.
(a) Describe the key steps the student should follow to obtain an accurate titre. [4]
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- Use a pipette to measure exactly 25.0 cm³ of NaOH into a conical flask [1]
- Add a few drops of indicator (e.g. methyl orange or phenolphthalein) [1]
- Fill a burette with HCl and record the start volume; add acid gradually with swirling [1]
- Stop adding acid at the end point (colour change of indicator); record the final burette reading to calculate the titre [1]
Question 9: Required Practical — Accurate Titration ⭐🔬 Extended Response
6 marksA student needs to determine the exact concentration of a sodium hydroxide solution by titrating it against a standard solution of hydrochloric acid (0.10 mol/dm³).
(a) Describe the full method the student should use to obtain accurate and reliable results, explaining how to improve the precision of the titration. [6]
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Level 3 (5–6 marks): Detailed, coherent method covering setup, procedure, and how to achieve concordant results. Precision techniques are explicitly described.
Level 2 (3–4 marks): Mostly correct method including the key equipment and steps, but may lack detail on obtaining reliable/concordant results.
Level 1 (1–2 marks): Basic statements about mixing acid and alkali, lacking procedural detail or correct equipment names.
Indicative content:
- Use a pipette (+ pipette filler) to transfer exactly 25.0 cm³ of NaOH into a clean conical flask
- Add 2–3 drops of a suitable indicator (e.g. methyl orange)
- Fill a burette with the standard HCl; record the initial reading
- Perform a rough titration first — add acid quickly to estimate the end point
- Repeat, adding acid dropwise near the end point for precision
- Record the titre; repeat until you obtain at least two concordant results (within 0.10 cm³); calculate a mean from concordant values only