HSC Exam Practice

Sample response. A precipitate is an insoluble solid that forms when two aqueous ionic solutions are mixed and the ion product Q for a particular ionic combination exceeds the solubility product K_sp: precipitation occurs when Q > K_sp.

Marking notes. 1 mark for defining precipitate as an insoluble solid formed in solution; 1 mark for the condition Q > K_sp (or equivalent: when concentration of ions in solution exceeds the saturation limit). Do not penalise if student omits Q notation but correctly states that ion concentrations exceed the solubility limit.

Sample response. The spectator ions are K⁺(aq) and NO₃⁻(aq). They are described as spectators because they are present in solution on both the left-hand (reactant) and right-hand (product) sides of the full ionic equation in identical form, charge, and state — they do not undergo any change and do not participate in the net chemical reaction. The net event is Pb²⁺(aq) + 2I⁻(aq) → PbI₂(s), from which K⁺ and NO₃⁻ are entirely absent.

Marking notes. 1 mark for correctly identifying both spectator ions (K⁺ and NO₃⁻); 1 mark for explaining that they appear identically on both sides of the full ionic equation; 1 mark for stating they do not participate in the net chemical reaction / are not consumed or produced.

Sample response. The molecular equation represents all reactants and products as complete formula units (e.g. 2KI(aq) + Pb(NO₃)₂(aq) → PbI₂(s) + 2KNO₃(aq)), including spectator ions as part of intact compounds. The net ionic equation shows only the species that actually participate in the reaction: all soluble ionic compounds are split into their constituent aqueous ions, the precipitate remains as its formula unit with state symbol (s), and spectator ions are removed (e.g. Pb²⁺(aq) + 2I⁻(aq) → PbI₂(s)).

Marking notes. 1 mark for distinguishing that molecular equation uses intact formula units for all species including spectator ions; 1 mark for explaining that net ionic equation splits aqueous species into ions; 1 mark for explaining that net ionic equation omits spectator ions. A correct example of each equation earns the relevant mark even without explicit verbal description.

Sample response. Within NAGSAG, the “S” rule states that sulfate salts are mostly soluble — not universally so. Ba²⁺ is one of the named exceptions: BaSO₄ (along with PbSO₄ and the sparingly soluble CaSO₄) is insoluble. The “mostly soluble” language signals that Ba²⁺, Pb²⁺, and Ca²⁺ must be memorised as exceptions. The underlying reason is that the lattice energy of BaSO₄ is very high (the large Ba²⁺ and SO₄²⁻ ions form a particularly stable lattice), outweighing hydration energy and driving the equilibrium strongly towards the undissolved solid.

Marking notes. 1 mark for identifying that NAGSAG’s S-rule says “mostly soluble” (not all); 1 mark for naming Ba²⁺ as a specific exception; 1 mark for explanation (high lattice energy, or equivalent: Ksp very low, equilibrium lies far left). Accept “Ba²⁺, Pb²⁺, Ca²⁺ are exceptions” for the second mark.

Sample response. Step 1: Identify all four ions present in solution when the two reactant solutions are mixed (two cations and two anions). Step 2: Identify the two possible new ionic combinations — each cation paired with the other solution’s anion. Step 3: Apply NAGSAG to each possible product to determine which (if either) is insoluble; the insoluble product is the precipitate. Step 4: Write the molecular equation (if precipitation occurs), then the full ionic equation (splitting all aqueous species), then cancel spectator ions to obtain the net ionic equation; verify charge balance on both sides of the net ionic equation.

Marking notes. 1 mark per step described correctly. Award 4 marks for all four steps in logical order; 3 marks for three steps; etc. Must include verification of charge balance for full Step 4 mark.

Sample response. A water treatment plant adds a soluble salt containing an anion that forms an insoluble compound with Pb²⁺. Sodium carbonate (Na₂CO₃) is a suitable choice: CO₃²⁻ combines with Pb²⁺ to form lead(II) carbonate, an insoluble precipitate (NAGSAG: carbonates are generally insoluble except with Group 1 or NH₄⁺). Net ionic equation: Pb²⁺(aq) + CO₃²⁻(aq) → PbCO₃(s). The precipitate settles or is filtered, removing Pb²⁺ from the treated water. The Na⁺ counter-ion is a spectator — it remains dissolved and is harmless. The precipitant must also be verified not to remove other essential ions (Ca²⁺ co-precipitation is a consideration).

Marking notes. 1 mark for naming a correct precipitant (Na₂CO₃, Na₃PO₄, or Na₂S — all acceptable with correct chemistry); 1 mark for naming the precipitate and stating it is insoluble; 1 mark for a correct net ionic equation (with state symbols); 1 mark for explaining how the precipitate is removed (filter/settle) and/or for explaining why the counter-ion (Na⁺) does not interfere.

Part (a) — 2 marks. Both sources show a linear (directly proportional) increase in CaCO₃ deposited as initial Ca²⁺ concentration increases. At any given Ca²⁺ concentration, Murrumbidgee River water deposits a greater mass of CaCO₃ per litre than Hawkesbury River water. [1 mark for “linear/directly proportional increase”; 1 mark for the comparative statement with correct direction.]

Part (b) — 2 marks. From the graph, reading between 60 and 90 mg/L Ca²⁺: at 60 mg/L the Murrumbidgee line gives approximately 30 mg/L CaCO₃; at 90 mg/L approximately 45 mg/L. By interpolation at 75 mg/L: (30 + 45)/2 ≈ 37.5 mg/L CaCO₃. Accept readings within ±3 mg/L. [1 mark for correct interpolation method; 1 mark for answer in range 34–41 mg/L.]

Part (c) — 3 marks. Net ionic equation: Ca²⁺(aq) + CO₃²⁻(aq) → CaCO₃(s). Charge check: +2 + (−2) = 0 = 0 ✓. NAGSAG: carbonates (CO₃²⁻) are generally insoluble; Ca²⁺ is not Group 1 or NH₄⁺, so CaCO₃ is insoluble → precipitate forms. Heating promotes reaction because CO₂ dissolved in the water is expelled when water is heated (gases are less soluble at higher temperatures), shifting CO₂(aq) ⇌ CO₂(g) equilibrium to the right. This depletes H⁺ and CO₂ while increasing CO₃²⁻ concentration, driving the precipitation of CaCO₃. [1 mark net ionic equation + charge balance; 1 mark NAGSAG application; 1 mark CO₂ expulsion / Le Chatelier explanation.]

Part (a) — 1 mark. Ag⁺(aq) + Cl⁻(aq) → AgCl(s). State symbols required. Charge balance: +1 + (−1) = 0 = 0 ✓.

Part (b) — 3 marks. n(AgCl) = 0.7175 / 143.32 = 5.006×10⁻³ mol [1 mark]. Since AgCl : Cl⁻ = 1:1, n(Cl⁻) = 5.006×10⁻³ mol [1 mark]. c(Cl⁻) = 5.006×10⁻³ / 0.2500 = 0.02002 mol/L [1 mark]. Accept 0.0200 mol/L or 2.00×10⁻² mol/L.

Part (c) — 2 marks. Convert: c(Cl⁻) = 0.02002 mol/L × 35.45 g/mol × 1000 mg/g = 709.5 mg/L. This exceeds the guideline of 250 mg/L, so the sample does not comply. [1 mark for correct calculation; 1 mark for clear compliance decision with units.]

Sample response. Solubility rules, codified in the NAGSAG framework, allow chemists to predict whether a specific ionic compound will form an insoluble solid when two solutions are mixed. This predictive power is directly applied in water treatment in two contexts: removal of heavy metals (e.g. dissolved Pb²⁺ in older Sydney Water distribution systems) and removal of unwanted mineral ions causing water hardness (e.g. CaCO₃/CaSO₄ in Murray-Darling Basin irrigation systems). In both cases, the chemist selects an anion that forms an insoluble compound with the target cation while remaining soluble with all other ions present. For Pb²⁺ removal, adding Na₂CO₃ introduces CO₃²⁻: by NAGSAG, carbonates are generally insoluble unless paired with Group 1 or NH₄⁺, so PbCO₃(s) precipitates [net ionic: Pb²⁺(aq) + CO₃²⁻(aq) → PbCO₃(s)]. The Na⁺ ions are spectators. The precipitate is then removed by filtration or sedimentation. For water hardness, calcium ions in the Murray-Darling Basin precipitate as CaCO₃ when lime (Ca(OH)₂) raises pH and shifts the carbonate equilibrium [Ca²⁺(aq) + CO₃²⁻(aq) → CaCO₃(s)]. The selectivity of the process depends on the K_sp values of possible products: engineers select the precipitant whose target-ion compound has the lowest K_sp, ensuring precipitation is most complete. Net ionic equations are essential because they isolate the essential chemistry from spectator ions, allowing the same equation to apply regardless of the counter-ions in the source water. Without solubility rules, it would be impossible to design selective treatment steps that remove one ion without disturbing other desirable species in the water.

Marking notes. 1 mark — explains NAGSAG as a framework for predicting solubility/precipitation. 1 mark — states the design principle: choose anion that forms insoluble product with target cation but soluble products with other ions. 1 mark — names at least one Australian context with correct target ion identified (Pb²⁺ in Sydney pipes; Ca²⁺/Mg²⁺ in Murray-Darling; CaCO₃ water hardness; accept any other specific Australian water quality context). 1 mark — second distinct Australian context with correct target ion. 1 mark — at least one correct net ionic equation with state symbols and verified charge balance. 1 mark — second correct net ionic equation or correct explanation of spectator ions in one of the contexts. 1 mark — explicit reference to K_sp as the criterion for selecting the most effective precipitant (lower K_sp → more complete precipitation). 1 mark — demonstrates analytical understanding of how the design process works: testing both possible products against NAGSAG, verifying that counter-ions remain dissolved, confirming treatment achieves guideline concentrations.