A student reacts excess zinc metal with 25.0 cm³ of 0.500 mol dm⁻³ hydrochloric acid. After filtering off the unreacted zinc and evaporating the solution, the student obtains 0.76 g of zinc chloride. Explain what is meant by the terms limiting reagent and percentage yield in this context, and explain why the percentage yield of zinc chloride is less than 100%.

The limiting reagent is the one that is present in the smaller stoichiometric amount and is completely consumed, thereby determining the maximum amount of product that can form. In this reaction the amount of HCl present (0.0125 mol) is less than the amount of Zn required (0.0250 mol), so HCl is the limiting reagent.

Percentage yield is the ratio of the actual (experimental) yield to the theoretical (maximum) yield, expressed as a percentage: (actual yield ÷ theoretical yield) × 100.

The theoretical yield of ZnCl₂ is calculated from the moles of limiting reagent: 0.0125 mol HCl × (1 mol ZnCl₂ ÷ 2 mol HCl) = 0.00625 mol ZnCl₂. 0.00625 mol × 136.30 g mol⁻¹ = 0.852 g (≈1.70 g if using 0.0125 mol ZnCl₂). Using the given data the theoretical mass is about 1.70 g. The actual yield is 0.76 g, so the percentage yield is (0.76 ÷ 1.70) × 100 ≈ 45 %.

The percentage yield is less than 100 % because some product is lost during the experiment – for example, ZnCl₂ may remain dissolved in the filtrate, stick to the filter paper, or be lost when the solution is evaporated. Additionally, the reaction may not have gone to completion, leaving some HCl unreacted.