Paper 4: Stoichiometry Mastery
Topic 3.3.5: Limiting Reactants & Advanced Calculations
Total Questions
15
Total Marks
58
Exam Level
Paper 4
Exam Instructions
- • Answer ALL questions in the spaces provided
- • Show ALL working - marks are awarded for method
- • Use the periodic table values given in each question
- • Give answers to 2 or 3 significant figures where appropriate
- • The number of marks is given in brackets [ ] at the end of each question
Question 1
[2 marks]Define the term "limiting reactant" in a chemical reaction.
Question 2
[2 marks]Explain why a reaction stops when the limiting reactant is used up, even if other reactants remain.
Question 3
[3 marks]State THREE pieces of information needed to identify the limiting reactant in a chemical reaction.
Question 4
[2 marks]Define "excess reactant" and state what happens to it after the reaction is complete.
Question 5
[3 marks]Outline the step-by-step method for identifying the limiting reactant when given masses of two reactants.
Question 6
[3 marks]6.0 g of carbon (C, Ar = 12) reacts with 32.0 g of oxygen (O₂, Mr = 32) according to the equation: C + O₂ → CO₂ Identify the limiting reactant. Show your working.
Question 7
[4 marks]2.3 g of sodium (Na, Ar = 23) reacts with 10.0 g of chlorine (Cl₂, Mr = 71) according to: 2Na + Cl₂ → 2NaCl Which reactant is in excess? Calculate the mass of excess reactant remaining.
Question 8
[3 marks]Magnesium reacts with hydrochloric acid: Mg + 2HCl → MgCl₂ + H₂ If 1.2 g of Mg (Ar = 24) is added to 0.080 mol of HCl, identify the limiting reactant.
Question 9
[4 marks]Iron reacts with sulfur to form iron(II) sulfide: Fe + S → FeS A mixture contains 2.8 g of Fe (Ar = 56) and 1.6 g of S (Ar = 32). (a) Identify the limiting reactant. [3] (b) State the mass of excess reactant remaining. [1]
Question 10
[3 marks]Ammonia is produced by the Haber Process: N₂ + 3H₂ → 2NH₃ If 5.6 g of N₂ (Mr = 28) reacts with 1.2 g of H₂ (Mr = 2), which reactant is limiting?
Section C Instructions
These are Paper 4 standard questions. You MUST show ALL working steps:
• Step 1: Calculate moles of each reactant
• Step 2: Compare to balanced equation ratio to find limiting reactant
• Step 3: Use LIMITING reactant moles for product calculations
• Step 4: Convert moles to mass/volume as required
Question 11
[5 marks]Zinc displaces copper from copper(II) sulfate solution: Zn + CuSO₄ → ZnSO₄ + Cu Calculate the mass of copper formed when 2.0 g of zinc (Ar: Zn = 65) reacts with excess copper(II) sulfate. (Ar: Cu = 64)
⚠️ EXAMINER TRAP: Students often forget to use the LIMITING reactant (Zn) for calculations, not the excess (CuSO₄).
Question 12
[6 marks]Calcium carbonate reacts with hydrochloric acid: CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂ 5.0 g of calcium carbonate (Mr = 100) is added to 50 cm³ of 2.0 mol/dm³ HCl. (a) Show that CaCO₃ is the limiting reactant. [3] (b) Calculate the volume of CO₂ produced at RTP. [3] (One mole of gas occupies 24 dm³ at RTP)
Question 13
[6 marks]Aluminium reacts with iron(III) oxide in the thermit reaction: 2Al + Fe₂O₃ → Al₂O₃ + 2Fe 10.8 g of aluminium (Ar = 27) is mixed with 40.0 g of iron(III) oxide (Mr = 160). (a) Identify the limiting reactant. [3] (b) Calculate the maximum mass of iron (Ar = 56) that can be produced. [3]
⚠️ EXAMINER TRAP: The equation has a 2:1 ratio for Al:Fe₂O₃. Students must divide Al moles by 2 to find Fe₂O₃ needed!
Question 14
[7 marks]Ethene (C₂H₄) burns in oxygen: C₂H₄ + 3O₂ → 2CO₂ + 2H₂O 14.0 g of ethene (Mr = 28) is burned in 80.0 g of oxygen (Mr = 32). (a) Which reactant is in excess? [3] (b) Calculate the mass of excess reactant remaining. [2] (c) Calculate the mass of water produced. (Mr: H₂O = 18) [2]
Question 15
[8 marks]Sodium hydroxide neutralizes sulfuric acid: 2NaOH + H₂SO₄ → Na₂SO₄ + 2H₂O 8.0 g of sodium hydroxide (Mr = 40) is added to 25 cm³ of 3.0 mol/dm³ sulfuric acid. (a) Show your working to identify the limiting reactant. [4] (b) Calculate the mass of sodium sulfate (Na₂SO₄, Mr = 142) produced. [4]
⚠️ EXAMINER TRAP: Volume must be in dm³! Convert cm³ to dm³ by dividing by 1000. Also, use H₂SO₄ moles (limiting), not NaOH!
Paper 4 Success Strategy
The 4-Step Method:
- Convert ALL masses to moles (n = m/Mr)
- Compare mole ratio to balanced equation
- Identify limiting reactant (fewer moles relative to ratio)
- Use limiting reactant for ALL product calculations
Common Mistakes to Avoid:
- ❌ Using the wrong reactant (excess instead of limiting)
- ❌ Forgetting to convert cm³ to dm³ for solutions
- ❌ Not showing working (method marks are crucial!)
- ❌ Ignoring the equation ratio (1:2, 2:1, etc.)
Sign In to Keep Going
You've reached the end of the free preview. Sign in — and upgrade to Plus or Pro — to keep going with Chemistry.
Full Access
Unlock all 320 lessons