Neutralising an Acid Spill
A 2.0 L bottle of hydrochloric acid spills in a laboratory prep room. The label reads 0.50 mol/L. The teacher has two neutralising agents on the shelf: solid sodium bicarbonate (NaHCO3, M = 84 g/mol) and solid sodium hydroxide (NaOH, M = 40 g/mol). The spill must be neutralised before it reaches a floor drain 3 m away. The teacher has 50 g of each solid available and wants to use the minimum mass that will fully neutralise the acid, while avoiding adding excess strong base to the floor.
1. Calculate the moles of HCl spilled and the mass of each neutraliser needed for complete neutralisation.
Read the worked answer
Moles HCl = 0.50 mol/L x 2.0 L = 1.0 mol. NaOH reacts 1:1 with HCl: moles NaOH needed = 1.0 mol, mass = 1.0 x 40 = 40 g. NaHCO3 reacts 1:1 with HCl (NaHCO3 + HCl -> NaCl + H2O + CO2): moles needed = 1.0 mol, mass = 1.0 x 84 = 84 g. So 40 g NaOH or 84 g NaHCO3 is needed.
2. The teacher only has 50 g of each solid. Which neutraliser can fully neutralise the spill, and which is the safer choice? Justify with numbers.
Read the worked answer
NaOH: 50 g available, 40 g needed, so 50 g is enough. But 10 g excess NaOH (0.25 mol) would be left as strong base on the floor, which is corrosive. NaHCO3: 50 g available, 84 g needed, so 50 g is not enough. It would neutralise only 50/84 = 0.60 mol of the 1.0 mol, leaving 0.40 mol HCl unreacted. Neither is ideal. The safer judgement: use the 50 g NaOH but add it carefully in portions and check pH, or combine approaches. Using NaOH with monitoring gives complete neutralisation; using NaHCO3 alone leaves acid, which is worse near a drain.