Module 2 mastery means solving a 5-step problem without losing the thread. This lesson chains all three inquiry questions together in exam-style problems — the same kind you'll face in your assessment tasks.
Understand the core concepts covered in this lesson.
Apply your knowledge to solve problems and explain phenomena.
Evaluate and analyse scientific information and data.
Use the PDF for classwork, homework or revision. It includes key ideas, activities, questions, an extend task and success-criteria proof.
A chemist reacts a 92% pure sample of Na₂CO₃ with HCl to produce CO₂ gas. If she mixes up the order of operations — applying the stoichiometry first, then correcting for purity afterwards — what would happen to her final answer? And how many distinct "steps" do you think are involved in going from a concentration of HCl to a volume of gas produced?
Type your initial thoughts below:
Record your ideas in your workbook.
🔗 Chained Problems
Calculate n(H₂SO₄) in the solution.
Using the mole ratio, determine n(H₂) produced.
Calculate the volume of H₂ produced at RTP.
Calculate the mass of Zn consumed.
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Answer in workbook.
Calculate the mass of pure Fe₂O₃ in 500 g of ore.
Calculate the theoretical yield of Fe from this sample. (Fe = 55.845, O = 15.999)
The actual yield of Fe collected is 245 g. Calculate the percentage yield.
Give one reason why the actual yield is less than theoretical.
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Answer in workbook.
Find the concentration of NaOH in the prepared solution.
Find the mass of pure NaOH in the full 250 mL solution.
Calculate the percentage purity of the commercial NaOH sample.
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Answer in workbook.
📋 Self-Assessment Checklist
Wrong: The mole is a measure of mass.
Right: The mole is a measure of amount of substance; one mole contains Avogadro's number of particles.
Pay close attention to reaction conditions and reagents. These determine the products formed.
Tick each item when you can do it without referring to notes. Be honest — this is your own study tool.
💡 Exam Strategy Tips
Even if a question seems straightforward, write and balance the equation first. The mole ratio is hidden in the coefficients — you can't access it without the equation.
In NSW exams, working is marked. A correct answer with no working gets no part-marks if wrong. Write: given → formula → substitution → answer with units.
Write units after every number throughout your calculation. Mol, g/mol, g, L, mol/L. Units tell you when you've made an error — mass ÷ mass gives a dimensionless ratio, not moles.
After calculating, ask: is this a reasonable quantity? If you get 1200 g of product from 0.01 mol of reactant, something is wrong. Order-of-magnitude checks catch arithmetic errors.
If a reactant is impure, correct for purity BEFORE calculating moles. m(pure) = m(sample) × (% purity ÷ 100). Never apply purity after the stoichiometry — it's already too late.
Concentration calculations require volumes in litres. The habit of immediately writing "V = ___ mL = ___ L" before any calculation will eliminate one of the most common error sources.
At the start of this lesson, you thought about what happens when purity is applied in the wrong order, and how many steps are in a chained stoichiometry problem.
Purity must be applied before stoichiometry — you need the mass of pure reactant before you can calculate moles. A full chained problem typically involves 5–7 steps: convert concentration to moles, apply purity if needed, use the mole ratio, convert moles to the required quantity (mass, volume, or concentration). Keeping track of what you know and what you need at each step is the key to solving these problems without losing the thread.
Reflect: how did your initial thinking compare to what you've learned?
Write a reflection in your workbook.
5 random questions from a replayable lesson bank — feedback shown immediately
✍️ Exam-Style Questions
Q1. A student dissolves 1.06 g of impure Na₂CO₃ (92.0% pure, MM = 105.99) in water and reacts it with excess HCl: Na₂CO₃ + 2HCl → 2NaCl + H₂O + CO₂. (a) Find the mass of pure Na₂CO₃ in the sample. (b) Calculate the theoretical volume of CO₂ at STP. 4 MARKS
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Answer in workbook.
Q2. 30.0 mL of 0.300 mol/L Ca(OH)₂ is mixed with 50.0 mL of 0.250 mol/L HCl: Ca(OH)₂ + 2HCl → CaCl₂ + 2H₂O. (a) Identify the limiting reagent. (b) Calculate the mass of CaCl₂ formed. (Ca = 40.078, Cl = 35.453) 5 MARKS
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Answer in workbook.
Q3. An ore sample (85.0% Fe₂O₃ by mass) is reacted with excess CO in a blast furnace: Fe₂O₃ + 3CO → 2Fe + 3CO₂. The actual mass of Fe collected from 300.0 g of the ore is 148 g. (a) Calculate the theoretical yield of Fe. (b) Calculate the percentage yield. (Fe = 55.845, O = 15.999) 5 MARKS
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Answer in workbook.
Q4. A chemist calculates that 0.0250 mol of NaOH should react with 0.0250 mol of HCl (1:1 ratio). After mixing 25.0 mL of 1.00 mol/L NaOH with 25.0 mL of 1.00 mol/L HCl, she evaporates the solution and finds 1.38 g of NaCl instead of the theoretical 1.46 g. (a) Calculate the percentage yield of NaCl. (b) The chemist suggests two possible explanations: (i) some NaCl was lost when evaporating the solution, or (ii) the HCl solution was slightly less concentrated than 1.00 mol/L. For each explanation, identify whether it represents a random or systematic error and predict the effect on all future trials. (Na = 22.990, Cl = 35.453) 5 MARKS
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Answer in workbook.
HCl has smaller n÷coeff value (6.25×10⁻³ < 9.00×10⁻³) → HCl is the limiting reagent
(b) Ratio HCl:CaCl₂ = 2:1; n(CaCl₂) = 1.25×10⁻² ÷ 2 = 6.25×10⁻³ mol MM(CaCl₂) = 110.98; m = 6.25×10⁻³ × 110.98 = 0.694 g(b) Explanation (i) — NaCl lost during evaporation: this is a random error if the amount lost varies between trials. It could also be systematic if the student consistently loses the same amount (e.g., always stops heating too early). It would generally give a yield below 100% in every trial, with the magnitude varying randomly.
Explanation (ii) — HCl less concentrated than 1.00 mol/L: this is a systematic error. Every trial uses the same solution; every trial would have the same deficit in HCl moles. The theoretical yield calculated from 0.0250 mol would be wrong in the same way every time — the % yield would be consistently below the true value in a predictable, reproducible way.
Tick when you've finished all chain problems and the checklist.