Chemical Stoichiometry
Mole ratios, limiting reagents, and reaction yields
Stoichiometry uses the coefficients of a balanced chemical equation to calculate the amounts of reactants consumed and products formed. The mole ratio between any two substances equals the ratio of their coefficients. The limiting reagent is completely consumed first, determining the maximum product. Excess reagent remains after the reaction. Theoretical yield is the maximum possible product mass; actual yield is always less due to incomplete reactions or side reactions.
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Sign in →What is Chemical Stoichiometry?
Chemical stoichiometry is the science of counting particles in chemical reactions. When substances react, atoms rearrange — but atoms are never created or destroyed. A balanced equation shows the exact ratio of particles involved. Think of it like a recipe: to make one batch of cookies you need a specific ratio of flour to eggs. In chemistry, the coefficients in a balanced equation are that recipe — if the equation reads 2A + B → AB₂, you need two parts A for every one part B. When one ingredient runs out first — the limiting reagent — the reaction stops, just like running out of eggs stops cookie-making even if you have plenty of flour. The excess reagent is whatever is left over. This simulation lets you drag atom counters, watch balanced reactions unfold particle-by-particle, and discover firsthand why the coefficient ratio controls how much product you can make. The core skill is reading the balanced equation's ratio, not mole-level calculations.
Parameters explained
- Moles A(mol)
- Moles A controls the amount of the first reactant available before the reaction begins. The HTML slider runs from 0 to 50, and the simulation displays that as 0.0 to 5.0 mol, so each slider step represents 0.1 mol. Increasing Moles A gives the particle model more A units to match against Reactant B according to the current balanced equation preset. If A is limiting, raising it can increase product formation. If B is limiting, raising A only leaves more A particles unreacted, which helps students distinguish total amount from the limiting reagent.
- Moles B(mol)
- Moles B controls the supply of the second reactant. Like Moles A, the slider uses 0 to 50 raw steps and displays them as 0.0 to 5.0 mol, giving fine control over the reactant ratio. Changing Moles B while holding Moles A steady is a direct way to test which substance runs out first. In the NH₃ preset, for example, B represents the reactant needed in a larger coefficient ratio. Students can compare the leftover particles after the reaction to the balanced equation and see why the smaller displayed mole amount is not always the limiting reagent.
- Reaction Progress(%)
- Reaction Progress scrubs the visualization from 0% to 100% complete without changing the starting amounts. At 0%, students see only the initial reactant particles. As the slider moves upward, matching groups of A and B are consumed and product particles appear according to the selected preset's balanced equation. Near 100%, the limiting reagent is used up and the excess reagent remains visible. This control is useful for pausing the reaction midway, asking students to count what has reacted so far, and connecting particle-level changes to the conservation of atoms across the whole reaction.
Common misconceptions
MisconceptionThe reactant you start with more of is always the limiting reagent.
CorrectThe limiting reagent is whichever substance runs out first based on the mole ratio in the balanced equation — not whichever has fewer total moles. For example, in 2H₂ + O₂, you need 2 moles of H₂ for every 1 mole of O₂. If you have 3 mol H₂ and 2 mol O₂, H₂ is limiting even though it seems like you have more of it, because the reaction needs twice as much H₂ relative to O₂.
MisconceptionWhen the reaction finishes, all the reactants are used up.
CorrectOnly the limiting reagent is completely consumed. The excess reagent still has leftover particles after the reaction stops. Real lab products often contain unreacted starting materials. The simulation shows these leftover particles explicitly so you can see which reactant had extra.
MisconceptionAdding extra of one reactant always makes more product.
CorrectIf the other reactant is already the limiting one, dumping in more of the first reactant just leaves more leftover — it does not make more product. To get more product, you need more of the limiting reactant (or more of both, in the right ratio from the balanced equation). This is the same as adding more flour will not bake more cookies if you have run out of eggs.
MisconceptionBalancing an equation changes the chemical formula of the substances.
CorrectBalancing only adjusts the coefficients in front of each formula — never the subscripts inside the formula. Changing subscripts would create a different substance entirely. The formulas stay the same; you are just deciding how many of each molecule is needed to conserve atoms on both sides of the equation.
How teachers use this lab
- Choose the H₂O preset, then set Moles A to 20 and Moles B to 30. Ask students to predict the limiting reagent before moving Reaction Progress, then verify by counting leftover particles in the visualization.
- Choose the CO₂ preset, then set Moles A to 10 and Moles B to 20. After moving Reaction Progress to 100%, ask students to identify the excess reagent and explain how the balanced equation coefficients determine the result.
- Compare the H₂O, NH₃, and CO₂ presets while recording the displayed Moles A and Moles B values. Students explain how changing the equation changes which particle groups can combine, even when the slider controls are the same.
- Introduce the limiting reagent concept using the cookie-baking analogy: molesA = flour batches, molesB = egg sets. Set values so one runs out first and have students articulate why the recipe stopped without using chemistry vocabulary initially.
- Have students set Moles A low and Moles B high, observe the excess at 100% Reaction Progress, then reverse the amounts. Ask whether product amount changed and why. This reinforces that yield depends on the limiting reagent, not total material.
Frequently asked questions
What does 'mole' mean, and why do chemists use it?
A mole is just a very large counting number — about 602 sextillion particles. Chemists use it because individual atoms and molecules are far too small to count one by one or weigh individually. The mole lets scientists connect the measurable mass of a substance in a lab to the number of atoms actually reacting. In this simulation, Moles A and Moles B represent proportional groups of particles so you can see the ratios without needing the full astronomical number.
Why does the reaction stop when the limiting reagent runs out?
Chemical reactions require the correct particles to collide with each other. Once the limiting reagent is completely consumed, there are no more of those particles available to collide with the excess reagent. The reaction has nowhere to go, so it stops. The remaining excess particles just sit there unreacted. This is directly observable in the simulation: you can see leftover particles of the excess reagent after the visualization ends.
What NGSS standards does this experiment address?
This simulation supports MS-PS1-2 (analyze and interpret data on properties of substances before and after substances interact to determine if a chemical reaction has occurred) and MS-PS1-5 (develop and use a model to describe how the total number of atoms is conserved when two or more substances combine or separate). The particle-counting visualization directly models conservation of atoms across a chemical reaction.
How is stoichiometry used in real life?
Stoichiometry is used everywhere manufacturing happens. Pharmaceutical companies calculate exact reagent amounts to synthesize medicines efficiently. Fuel engineers calculate air-to-fuel ratios for complete combustion in engines. Food scientists balance ingredient ratios in industrial baking. Even your body uses stoichiometric principles — enzymes catalyze reactions in precise molecular ratios to keep cellular chemistry running correctly.
How do I tell which reactant will run out first?
Look at the balanced equation to see how many of each particle are needed in the reaction's recipe. Compare that ratio to how much of each reactant you actually have. Whichever reactant runs out first when matching that recipe ratio is the limiting reactant. In the simulation, after pressing Play you can also count the leftover particles — the reactant that has none left was the limiting reactant. The other reactant, with leftovers, was in excess.