HSC Exam Practice

Sample response. Stoichiometry is the branch of chemistry that uses balanced equations to calculate quantities of reactants and products involved in chemical reactions. A balanced equation provides the mole ratio — the coefficients indicate the exact proportions in which substances react and are produced. These ratios act as conversion factors, allowing the moles of any species to be calculated from the known moles of any other species in the reaction.

Marking notes. 1 mark for a correct definition of stoichiometry referencing balanced equations and quantities. 1 mark for identifying that coefficients provide the mole ratio. 1 mark for explaining that the mole ratio is used as a conversion factor to relate moles of one species to another.

Sample response. A coefficient is the large number written in front of a chemical formula, indicating the number of moles of that substance in the reaction (e.g. the 2 in 2HCl). A subscript is the small number written within the formula indicating the number of atoms of that element in one formula unit (e.g. the 2 in H₂O). Coefficients can be changed when balancing because doing so only adjusts the number of moles of a substance without altering its chemical identity. Subscripts cannot be changed because doing so produces an entirely different substance — for example, changing H₂O to H₂O₂ produces hydrogen peroxide, not water.

Marking notes. 1 mark for a correct definition of coefficient (moles of substance, large number in front). 1 mark for a correct definition of subscript (atoms per formula unit, small number inside formula). 1 mark for explaining why coefficients can be changed (adjusts quantity only, not identity). 1 mark for explaining why subscripts cannot be changed (changes the chemical identity/formula of the substance).

Sample response. The Law of Conservation of Mass states that atoms are rearranged but never created or destroyed in a chemical reaction. This means the total number of each type of atom must be the same on both sides of the equation. A balanced equation ensures this condition is met. If an equation is unbalanced, it implies atoms have been created or destroyed, which would violate the law and misrepresent the actual reaction.

Marking notes. 1 mark for stating that atoms are rearranged but not created or destroyed. 1 mark for linking this to requiring equal atom counts on both sides. 1 mark for stating that an unbalanced equation implies a violation of conservation of mass.

Sample response. Mole ratio Al : Cl₂ : AlCl₃ = 2 : 3 : 2 (from coefficients). n(Cl₂) = 0.900 × (3÷2) = 1.35 mol. n(AlCl₃) = 0.900 × (2÷2) = 0.900 mol.

Marking notes. 1 mark for correctly stating the mole ratio 2:3:2. 1 mark for correct calculation of n(Cl₂) = 1.35 mol with working. 1 mark for correct calculation of n(AlCl₃) = 0.900 mol with working.

Sample response. The mole ratio is determined solely by the coefficients in the balanced equation, which reflect the fixed proportions in which atoms combine to form products. These proportions arise from the fixed atomic composition of the reactants and products and do not change with scale. Whether 1 mol or 1000 mol of reactant is used, the ratio of moles of one species to another remains constant because it is a property of the chemical reaction itself, not of the quantity of material.

Marking notes. 1 mark for explaining that the mole ratio comes from the coefficients/atomic composition, not from the quantity of material. 1 mark for stating that the ratio is scale-independent because it is a fixed property of the reaction.

Sample response. The student’s error is using subscripts (the small numbers inside the formula H₂) instead of coefficients (the large numbers in front of formulas) to determine the mole ratio. In 2H₂O, the subscript 2 in H₂ tells you there are two hydrogen atoms in one water molecule — it says nothing about the mole ratio between H₂ and O₂ in any reaction. The correct way to determine a mole ratio is to first write the balanced equation for the reaction in question (e.g. 2H₂ + O₂ → 2H₂O), then read the coefficients: the mole ratio H₂ : O₂ = 2 : 1.

Marking notes. 1 mark for identifying the specific error (using subscripts instead of coefficients). 1 mark for explaining what the subscript 2 in H₂ actually means (atom count within one molecule, not a mole ratio). 1 mark for stating the correct method (write the balanced equation; read the coefficients).

Sample response (a). n(NH₃) increases rapidly from 0 mol at t = 0, with the steepest rate in the first 4–6 minutes [1]. The rate slows progressively and the curve levels off (plateaus) at approximately 18 mol NH₃ by about 12 minutes, at which point the reaction has effectively reached completion [1].

Sample response (b). Ratio N₂ : NH₃ = 1 : 2. n(N₂) consumed = 18.0 × (1÷2) = 9.00 mol [1]. Ratio H₂ : NH₃ = 3 : 2. n(H₂) consumed = 18.0 × (3÷2) = 27.0 mol [1]. Check: started with 10.0 mol N₂ and 30.0 mol H₂; 9.00 mol N₂ consumed (1.00 mol remaining); 27.0 mol H₂ consumed (3.00 mol remaining). Both consistent with stoichiometric ratio [1].

Sample response (c). % N₂ reacted = (9.00 ÷ 10.0) × 100 = 90% [1]. Conservation of mass: total atoms in = N₂ + H₂ consumed = 9.00 mol N₂ × 28 g/mol + 27.0 mol H₂ × 2 g/mol = 252 + 54 = 306 g of atoms. 18.0 mol NH₃ × 17 g/mol = 306 g [1]. The total mass of products equals the total mass of reactants consumed, confirming the Law of Conservation of Mass is satisfied [1].

Marking notes. Part (a): 1 mark for correct description of trend (rapid increase then plateau); 1 mark for identifying ~12 min as the time at which the reaction effectively reaches completion. Part (b): 1 mark for n(N₂) = 9.00 mol with working; 1 mark for n(H₂) = 27.0 mol with working; 1 mark for checking consistency with initial quantities. Part (c): 1 mark for % = 90%; 1 mark for conservation of mass calculation (mass in = mass out); 1 mark for explicitly confirming conservation of mass is satisfied.

Sample response. The claim that mole ratios are “merely a mathematical convenience” is incorrect and misrepresents the physical basis of stoichiometry. At the particle level, the mole ratio reflects the fixed proportions in which atoms and molecules combine, as dictated by conservation of mass and the Law of Definite Proportions. For example, in 2H₂(g) + O₂(g) → 2H₂O(g), the 2 : 1 : 2 ratio means that at the molecular level, every single O₂ molecule requires exactly two H₂ molecules to form two H₂O molecules. The ratio arises from the electronic structure and bonding requirements of the atoms — it is physically determined, not arbitrary. In stoichiometric calculations, the mole ratio serves as a conversion factor: given n(H₂) = 4.00 mol, n(H₂O) = 4.00 × (2÷2) = 4.00 mol. This calculation is only meaningful because the ratio reflects an underlying physical reality about how atoms combine. In the Haber process (N₂ + 3H₂ → 2NH₃), knowing the 1 : 3 : 2 mole ratio is critical for industrial planning: engineers use it to determine the stoichiometric feedstock quantities and to calculate yield per pass. If the ratio were merely a convention, there would be no basis for predicting that 100 mol N₂ requires exactly 300 mol H₂. The stoichiometry enforces real physical constraints. A second example from Australian industry is the thermite reaction (3Fe₃O₄ + 8Al → 9Fe + 4Al₂O₃) used in railway weld repair. The mole ratio determines exactly how much Al powder is packed with the iron oxide, directly controlling the yield of molten iron produced. Too little Al would leave unreacted oxide; too much would waste material. The claim is therefore false: the mole ratio has direct physical meaning at both particle and macroscopic scales, and its application in stoichiometric calculations reflects real chemical constraints, not mathematical convenience.

Marking criteria (7 marks). [1] Correctly refutes the claim by stating the physical basis of mole ratios (atomic/molecular combining proportions from conservation of mass and definite proportions). [1] Explains the particle-level meaning of the ratio for a named reaction (e.g. 2H₂ + O₂ → 2H₂O: each O₂ needs two H₂ molecules). [1] Demonstrates correct use of a mole ratio in a stoichiometric calculation (formula applied with correct values). [1] First named industrial or significant reaction with the mole ratio applied correctly (Haber process or equivalent). [1] Second named reaction with mole ratio correctly applied (thermite, combustion, neutralisation — any valid second reaction). [1] Discusses why the ratio is scale-independent and physically determined, not arbitrary. [1] Reaches an explicit evaluative judgement clearly stating the claim is incorrect and integrating both particle-level and macroscopic evidence.