Limiting Reactant and Yield in a Neutralisation
A student mixes 100 cm³ of 0.50 mol/dm³ hydrochloric acid (HCl) with 100 cm³ of 0.30 mol/dm³ sodium hydroxide (NaOH) in a calorimeter. The reaction is HCl + NaOH → NaCl + H₂O. She expects the solution to become neutral, but after mixing she finds it is still acidic when tested with indicator. She wants to know which reactant limited the reaction, how much salt formed, and how much of the excess acid remains. Relative atomic masses: Na = 23, Cl = 35.5, H = 1, O = 16.
1. Calculate the moles of each reactant and identify the limiting reactant.
Read the worked answer
Moles = concentration × volume (in dm³). 100 cm³ = 0.100 dm³.
HCl: 0.50 × 0.100 = 0.050 mol. NaOH: 0.30 × 0.100 = 0.030 mol.
The equation shows a 1:1 ratio. NaOH (0.030 mol) is present in smaller amount and is fully consumed → NaOH is the limiting reactant. HCl is in excess, which explains why the mixture stays acidic.
2. Find the mass of NaCl formed and the moles (and concentration) of HCl left over in the combined 200 cm³ solution.
Read the worked answer
Salt formed = moles of limiting reactant = 0.030 mol NaCl.
Molar mass NaCl = 23 + 35.5 = 58.5 g/mol. Mass = 0.030 × 58.5 = 1.755 g.
HCl used = 0.030 mol, so HCl remaining = 0.050 − 0.030 = 0.020 mol.
Total volume = 200 cm³ = 0.200 dm³. Concentration of leftover HCl = 0.020 / 0.200 = 0.10 mol/dm³ - still acidic, confirming the indicator result.
3. The student claims the solution is neutral because equal volumes were mixed. Explain the flaw and state what volume of 0.50 mol/dm³ HCl would exactly neutralise 100 cm³ of the 0.30 mol/dm³ NaOH.
Read the worked answer
Equal volumes do not mean equal moles: neutralisation requires equal moles, and the concentrations differ (0.50 vs 0.30 mol/dm³), so equal volumes give unequal moles. Neutrality depends on mole ratio, not volume.
For exact neutralisation, moles HCl must equal 0.030 mol. Volume = moles / concentration = 0.030 / 0.50 = 0.060 dm³ = 60 cm³ of the HCl. (Mixing 100 cm³ was an excess of 40 cm³.)