Limiting reagent and yield
When two substances react, one usually runs out first and limits how much product can form. The amount actually isolated compared with that is the yield.
Formulas and rules
- For each reactant: n / ν (moles divided by coefficient)
- The smallest value belongs to the limiting reagent
- Theoretical product from the limiting reagent
- Yield = actual mass / theoretical mass × 100%
Your turn
Three problems on this topic. Type numbers like 12.5 or 1,250 and elasticities with their sign.
Task
Burning methane: CH₄ + 2 O₂ → CO₂ + 2 H₂O. You start with 57 g of CH₄ and 8.8 g of O₂, and isolate 4.1 g of CO₂. Atomic masses (g/mol): H 1.01 · C 12.01 · O 16.00.
Which substance is the limiting reagent?
Theoretical mass of CO₂ in g (2 decimal places)
Yield in % (1 decimal place)
Numbers like 12.5 or 0.025, with as many decimals as asked. Masses and amounts count within 1%, pH values within ±0.02.
Worked example
Neutralisation: NaOH + HCl → NaCl + H₂O. You start with 75.7 g of NaOH and 31.6 g of HCl, and isolate 34.4 g of NaCl. Atomic masses (g/mol): H 1.01 · O 16.00 · Na 22.99 · Cl 35.45.
Worked solution
- 1.n(NaOH) = 75.7 / 40.00 = 1.8925 mol → divided by 1: 1.8925
- 2.n(HCl) = 31.6 / 36.46 = 0.8667 mol → divided by 1: 0.8667
- 3.The smaller value limits: HCl is the limiting reagent
- 4.n(NaCl) = 0.8667 × 1 = 0.8667 mol → m = 0.8667 × 58.44 = 50.65 g
- 5.Yield = 34.4 / 50.65 × 100 = 67.9%
Typical mistake: Taking the substance with the smaller mass or amount as limiting. What counts is the amount divided by its coefficient in the equation.
Common questions
Is the substance with the smaller mass limiting?
Not necessarily. What counts is the moles divided by the coefficient in the equation.