Chemistry • Year 11 • Module 2 • Lesson 11
Stoichiometry — Mole Ratios
Lock in the core vocabulary, the mole-ratio rule and the procedure for balancing equations before tackling harder questions.
1. Term–definition match
The definitions below are shuffled. In the right-hand column write the matching term from this list: stoichiometry, balanced equation, coefficient, subscript, mole ratio, conservation of mass, reaction ratio, conversion factor. 8 marks (1 each)
| # | Definition | Matching term |
|---|---|---|
| 1.1 | The branch of chemistry that uses balanced equations to calculate quantities of reactants and products. | |
| 1.2 | A chemical equation with equal numbers of each type of atom on both sides, satisfying conservation of mass. | |
| 1.3 | The large number written in front of a formula in a balanced equation, indicating the number of moles of that substance. | |
| 1.4 | The small number written within a chemical formula indicating how many atoms of that element are in one formula unit. | |
| 1.5 | The ratio of moles of one substance to another in a reaction, taken directly from the coefficients of the balanced equation. | |
| 1.6 | The law stating that atoms are neither created nor destroyed in a chemical reaction, so total mass is constant. | |
| 1.7 | Another name for the mole ratio — the exact proportions in which substances react and are produced. | |
| 1.8 | A ratio used to convert a quantity from one unit or substance to another; the mole ratio acts as one of these between species. |
2. True or false — with correction
Circle T or F for each statement. If the statement is false, write the corrected version on the line below it. 12 marks (1 T/F + 1 correction each)
2.1 Subscripts in a chemical formula give the mole ratio between different substances in a reaction. T / F
2.2 In the equation 2H2 + O2 → 2H2O, the mole ratio of H2 to O2 is 2 : 1. T / F
2.3 When balancing an equation, you may change subscripts inside formulas to make atom counts equal. T / F
2.4 An implied coefficient of 1 means there are zero moles of that substance in the reaction. T / F
2.5 The general formula n(wanted) = n(given) × coeff(wanted) ÷ coeff(given) uses coefficients from the balanced equation. T / F
2.6 The mole ratio changes if the same reaction is scaled up to industrial quantities. T / F
3. Fill-in-the-blank paragraph
Use the word bank to complete the passage. Each word is used once. 8 marks (1 per blank)
Word bank:
balanced · coefficients · conservation · conversion · mole ratio · products · rearranged · subscripts
The Law of ___________ of Mass states that atoms are ___________ in a chemical reaction — they are never created or destroyed. This means the total mass of ___________ always equals the total mass of reactants. For a chemical equation to be ___________, the number of atoms of each element must be equal on both sides. Only ___________ (the large numbers in front of formulas) may be changed when balancing — never the ___________ (the small numbers inside formulas). Once an equation is balanced, the coefficients define the ___________ for all species in the reaction. This ratio acts as a ___________ factor, allowing you to calculate moles of any substance given moles of another.
4. Function recall
Answer each question in 1–2 sentences using precise terms from the lesson. 8 marks (2 each)
4.1 What information in a balanced chemical equation gives you the mole ratio between two substances?
4.2 Why is it incorrect to use subscripts (rather than coefficients) to determine the mole ratio in a reaction?
4.3 State the general formula for finding the moles of a wanted substance given the moles of a known substance.
4.4 In the Haber process equation N2 + 3H2 → 2NH3, state the mole ratio of N2 to H2 to NH3.
5. Build a concept map
Draw labelled arrows between the six terms below to show how they connect. Each arrow must carry a linking phrase (e.g. “is found in”, “gives”, “must be balanced using”). Aim for at least 6 labelled arrows. 6 marks (1 per valid labelled arrow)
Supplied terms: balanced equation · coefficient · mole ratio · subscript · conservation of mass · moles of product.
6. Sequence the balancing steps
The five steps for balancing a chemical equation by inspection are listed below in the wrong order. Write the correct order (1–5) in the “Order” column. 5 marks (1 per correctly placed step)
| Order | Step description |
|---|---|
| Check all atom counts are equal on both sides and that coefficients are in the simplest whole-number ratio. | |
| Write the correct unbalanced equation with all formulas correct. | |
| Add or adjust coefficients in front of formulas — never change subscripts. Start with the most complex molecule, balance metals and non-metals before hydrogen, then balance oxygen last. | |
| Count atoms of each element on each side of the arrow. | |
| Identify which element is out of balance and adjust a coefficient to correct it, then recount. |
Q1 — Term–definition match
1.1 stoichiometry • 1.2 balanced equation • 1.3 coefficient • 1.4 subscript • 1.5 mole ratio • 1.6 conservation of mass • 1.7 reaction ratio • 1.8 conversion factor.
Q2 — True / false with correction
2.1 False. Subscripts show the atom ratio inside one formula unit or molecule. Coefficients in the balanced equation give the mole ratio between different substances in a reaction.
2.2 True. Coefficients are 2 (H2) and 1 (O2), giving a ratio of 2 : 1.
2.3 False. Only coefficients (large numbers in front of formulas) may be changed. Changing subscripts produces a different substance entirely (e.g. H2O becomes H2O2).
2.4 False. An implied coefficient of 1 means there is exactly 1 mole of that substance. A missing coefficient is always 1, not zero.
2.5 True.
2.6 False. The mole ratio is fixed by the balanced equation and holds at any scale — lab, pilot plant or industrial quantities. Only the absolute amounts of reactants and products change, not the ratio.
Q3 — Cloze paragraph
In order: conservation / rearranged / products / balanced / coefficients / subscripts / mole ratio / conversion.
Q4 — Function recall
4.1 The coefficients (the large numbers written in front of each formula) in the balanced equation give the mole ratio between any two substances in a reaction.
4.2 Subscripts tell you the number of atoms of an element within one formula unit (e.g. the 2 in H2 means two hydrogen atoms per molecule). They do not represent how many moles of that substance react or are produced. Using subscripts instead of coefficients gives a completely incorrect mole ratio.
4.3 n(wanted) = n(given) × coeff(wanted) ÷ coeff(given).
4.4 N2 : H2 : NH3 = 1 : 3 : 2 (read directly from the coefficients).
Q5 — Sample concept map
Correct maps should include arrows such as:
- balanced equation — contains → coefficient
- coefficient — defines → mole ratio
- mole ratio — used to calculate → moles of product
- conservation of mass — requires → balanced equation
- subscript — is NOT used for → mole ratio
- balanced equation — satisfies → conservation of mass
Award 1 mark per valid labelled arrow. Accept alternative valid connections with correct linking phrases.
Q6 — Correct balancing sequence
Step 1: Write the correct unbalanced equation. Step 2: Count atoms of each element on each side. Step 3: Identify which element is out of balance and adjust a coefficient. Step 4: Add or adjust coefficients (never change subscripts), starting with the most complex molecule. Step 5: Check all atom counts are equal and simplify the ratio.
Marking note: the order given in the lesson is 1→2→3→4→5 corresponding to the table rows: row 2 (write equation) = 1; row 4 (count atoms) = 2; row 5 (identify imbalance, adjust) = 3; row 3 (adjust coefficients systematically) = 4; row 1 (final check) = 5. Award 1 mark per row correctly ordered.