Theory — Limiting and Excess Reagents
Why One Reactant Runs Out First
When two reactants are mixed, they are rarely present in the exact ratio the balanced equation requires. The limiting reagent is the reactant that is used up first — it limits how much product can form. The other reactant is the excess reagent; some of it is left over when the reaction stops.
2. Convert each reactant mass to moles (mass ÷ molar mass).
3. Divide each by its coefficient — the smallest value is the limiting reagent.
4. Use the limiting reagent's moles to find the product.
Product Formed and Excess Remaining
The amount of product is calculated from the limiting reagent only. The excess remaining is found by subtracting the amount of excess reactant actually used from the amount supplied.
mass of product = moles of product × molar mass
excess used = moles of limiting × (excess coeff ÷ limiting coeff) × molar mass
excess remaining = excess supplied − excess used
Limiting Reagent
Runs out first; sets the maximum product. Found by the smallest moles ÷ coefficient.
Excess Reagent
Left over after the reaction. Excess remaining = supplied − used.
Percent Yield
Compares the actual product obtained to the theoretical maximum.
| Step | What you do |
|---|---|
| 1. Balance | Write the balanced equation |
| 2. Moles | mass ÷ molar mass for each reactant |
| 3. Compare | moles ÷ coefficient — smallest is limiting |
| 4. Product | from limiting reagent moles |
| 5. Excess | supplied − used |
Apparatus
The equipment a real limiting-reagent experiment uses. In the simulation these are modelled for you, but the readings correspond to what each instrument would measure.
Instructions — Running the Virtual Experiment
The Limiting Reagent bench lets you set two reactant masses and work out which is limiting; the Percent Yield section adds an actual yield. Record every reading in your lab report with screenshots.
Simulation — Limiting Reagents
Predict: which reactant is the limiting reagent?
Reaction & masses
You are given the reactant amounts and the experimental (actual) yield obtained in the lab. Work out the theoretical yield from the limiting reagent, then calculate the percent yield and enter it below to check your answer.
Reaction
Your calculation
Team Questions
Example Lab Report
Sample report demonstrating the expected format. Include labelled screenshots of the bench for each combination, and your full working for the limiting reagent and percent yield.
Limiting Reagents
Chemistry | Section: [Your Section] | Date: [Date]
Lab Members: [Names of all members present]
Purpose
To identify the limiting and excess reagents in a reaction from the masses of the reactants, to calculate the mass of product formed and the mass of excess reagent remaining, and to determine the percent yield.
Theory
The limiting reagent is the reactant that is consumed first and sets the theoretical yield. It is found by converting each reactant mass to moles, dividing by its coefficient, and choosing the smallest. The product is calculated from the limiting reagent; the excess remaining is the supplied amount minus the amount used.
Results (worked example: 2Mg + O₂ → 2MgO)
| Quantity | Value |
|---|---|
| Mass of Mg | 4.86 g (0.200 mol) |
| Mass of O₂ | 5.00 g (0.156 mol) |
| Mg ÷ 2 | 0.100 |
| O₂ ÷ 1 | 0.156 |
| Limiting reagent | Mg |
| MgO formed (0.200 mol × 40.30) | 8.06 g |
| O₂ used (0.100 mol × 32.00) | 3.20 g |
| O₂ remaining (5.00 − 3.20) | 1.80 g |
Percent yield example: if the actual MgO obtained is 7.00 g, percent yield = 7.00 / 8.06 × 100 = 86.8 %.
Analysis
Magnesium was limiting because its moles ÷ coefficient (0.100) was smaller than that of oxygen (0.156). All of the magnesium reacted, while 1.80 g of oxygen was left over. Increasing the oxygen further would not change the amount of MgO formed, because magnesium still limits the reaction.
Conclusion
The limiting reagent (magnesium) determined the theoretical yield of magnesium oxide, the excess oxygen remained unreacted, and the percent yield compared the actual product to that theoretical maximum. Identifying limiting and excess reagents is essential for predicting product amounts and using reactants efficiently in real processes.
Practice Questions
Show all work. Find the limiting reagent first, then the product and any excess. Molar masses: Mg 24.31, O₂ 32.00, MgO 40.30, Al 26.98, Cl₂ 70.90, AlCl₃ 133.33, Zn 65.38, HCl 36.46, ZnCl₂ 136.29 g/mol.