Neutralising an Acid Spill: The Safest Reagent Is Not the Smallest Mass
During a practical class a bottle tips over and 500 mL of 0.2 M hydrochloric acid spreads across a bench and drips into a shallow tray. The cupboard holds three solids that could neutralise it: sodium hydroxide pellets, sodium hydrogen carbonate powder, and quicklime (calcium oxide). The lab manual's instruction is to add the solid in small portions, stir, and stop when the mixture no longer fizzes; the final liquid is then tested with pH paper before it is poured down the sink. The balance reads to 0.1 g. On the same bench stands a bottle containing 250 mL of 0.5 M copper sulphate solution, and the assistant suggests transferring it to a cleaned iron can lying nearby. Assume complete reaction, a temperature of 25 °C, and that the hydrochloric acid is fully dissociated. Molar masses: NaOH = 40, NaHCO₃ = 84, CaO = 56 g/mol; molar volume of a gas at STP = 22.4 L.
1. Calculate the amount of hydrochloric acid spilled, the mass of sodium hydrogen carbonate needed to neutralise it, and the volume of gas released at STP.
Read the worked answer
Moles of HCl: n = C × V = 0.2 mol/L × 0.500 L = 0.100 mol
Reaction: NaHCO₃ + HCl → NaCl + H₂O + CO₂ Ratio is 1 : 1, so 0.100 mol of NaHCO₃ is required. Mass = 0.100 × 84 = 8.4 g (weighable to 0.1 g on the balance).
Gas: 0.100 mol of CO₂ × 22.4 L/mol = 2.24 L at STP
Why the 'stop when fizzing stops' rule works: as long as acid remains, each portion of bicarbonate produces CO₂, so the tray fizzes. When the acid is used up, the fizzing stops and any extra bicarbonate simply leaves a mildly alkaline solution. That gives the student a visible endpoint that needs no indicator, and it is the reason the manual specifies this reagent for a spill rather than a titration-style addition.
2. Calculate the mass of sodium hydroxide and of quicklime that would also remove the acid. Both masses are smaller than 8.4 g - argue which reagent should actually be used and why.
Read the worked answer
NaOH: NaOH + HCl → NaCl + H₂O, 1 : 1 Required = 0.100 mol × 40 = 4.0 g
CaO: CaO + 2HCl → CaCl₂ + H₂O, so CaO : HCl = 1 : 2 Required = 0.050 mol × 56 = 2.8 g
So the smallest mass (2.8 g) is quicklime and the largest (8.4 g) is bicarbonate - yet the bicarbonate is the correct choice:
- NaOH pellets are highly corrosive to skin and eyes, are deliquescent (they absorb water and become slippery, hard to handle), and dissolve with a strong exotherm that can spit acid out of the tray. A spill is exactly the situation where a corrosive strong base is least acceptable.
- CaO reacts violently with water to give Ca(OH)₂ (slaking), releasing a large amount of heat and producing a strongly alkaline slurry with fine dust; the reaction is fast and hard to control when added in 'small portions'.
- NaHCO₃ is mild, non-corrosive, reacts predictably, and is self-limiting through the fizzing endpoint. Its cost and extra 5.6 g of solid are irrelevant compared with safety and controllability.
The judgement tested here is that the minimum-mass answer is not the correct answer. Also verify the final liquid with pH paper: a faintly basic pH of about 8 means a slight excess of bicarbonate, which is harmless, whereas excess NaOH or CaO would leave pH 12–13 and must not go down the sink.
3. Should the 250 mL of 0.5 M copper sulphate solution be stored in the iron can? Support your answer with the reaction, a mass calculation, and the practical consequence.
Read the worked answer
No. Iron lies above copper in the reactivity series, so it displaces copper from the salt solution:
Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s)
Moles of CuSO₄ = 0.5 mol/L × 0.250 L = 0.125 mol
- Copper deposited = 0.125 × 63.5 ≈ 7.9 g
- Iron consumed = 0.125 × 56 = 7.0 g
Consequences: the blue colour fades and turns pale green as Fe²⁺ replaces Cu²⁺, a reddish-brown copper coating appears on the metal, and 7.0 g of the can's wall is eaten away - enough to perforate a thin can and spill the solution. Both the solution and the container are ruined.
Correct action: keep copper sulphate solution in a plastic or glass container (as it already is) and never in a vessel made of a metal more reactive than copper - iron, zinc, aluminium or magnesium. A copper or silver vessel would be chemically safe but is needlessly expensive. This is the same displacement principle as the thermite-type reactions and electroplating, applied to a storage decision.