General Chemistry  ·  Unit 09

Stoichiometry: Mole Ratios, Limiting Reactant, and Yield

Stoichiometry comes down to one idea: use the balanced equation to move through mole ratios, limiting reactant logic, and yield calculations, then convert to the unit the question wants.

What you'll learn

Use mole ratios from a balanced equation to convert between reactants and products. Identify the limiting reagent and calculate theoretical yield. Calculate percent yield and explain common sources of experimental error. Solve multi-step stoichiometry problems involving mass, moles, and gas volume.

9.1 Start Here: What Stoichiometry Is Actually Doing

The word stoichiometry (stoy-key-AH-meh-tree) sounds bigger than the idea really is. It is the math that tells you how much of each chemical is used or made in a reaction.

This is the unit where the mole work from Unit 07 and the balanced equations from Unit 08 finally work together.

Think of a recipe. If a cookie recipe says "use 2 cups of flour for every 1 cup of sugar," you can scale that ratio up or down.

A balanced chemical equation works the same way. The coefficients are the fixed recipe amounts for the reaction.

Example Equation 2 H2+O2→2 H2O

This means: 2 molecules of hydrogen gas react with 1 molecule of oxygen gas to make 2 molecules of water. The ratio 2 : 1 : 2 never changes — no matter how big or small the batch.

Particle model showing 2 H2 molecules plus 1 O2 molecule forming 2 H2O molecules PARTICLE MODEL H H H H 2H2 + O O O2 → O H H O H H 2H2O Mobile particle model showing 2 H2 molecules plus 1 O2 molecule forming 2 H2O molecules PARTICLE MODEL Reactants H H H H 2H2 + O O O2 Products O H H O H H 2H2O
Stoichiometry starts with the balanced particle ratio. This diagram shows why the coefficients become the fixed relationship you use later as a mole ratio: 2 parts H2, 1 part O2, and 2 parts H2O.
Reactant 1
2 mol H2

The coefficient says each batch needs 2 moles of hydrogen.

Reactant 2
1 mol O2

The same batch needs 1 mole of oxygen.

Product
2 mol H2O

That batch makes 2 moles of water.

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Big idea

  • The coefficients in a balanced equation tell you the mole ratio — the ratio of moles of every substance in the reaction.
  • This ratio is the heart of all stoichiometry problems.
  • If the equation is not balanced first, every later number is built on the wrong ratio.

9.2 Mole Ratios: The Conversion Tool That Drives the Whole Unit

A mole ratio (also called a stoichiometric factor) is a fraction you build from the coefficients in a balanced equation. You use it like a unit conversion — it lets you switch from moles of one substance to moles of another.

Notice what usually goes wrong here: grabbing numbers from subscripts or molar masses instead of the equation. Do not do that. The mole ratio comes only from the coefficients in the balanced equation.

General Stoichiometric Factor
mol of wanted substance mol of given substance = stoichiometric factor

From the equation N2 + 3 H2 → 2 NH3, you can write these mole ratios:

3 mol H21 mol N2 For every 1 mole of nitrogen used, 3 moles of hydrogen are used. 2 mol NH31 mol N2 For every 1 mole of nitrogen used, 2 moles of ammonia are made. 2 mol NH33 mol H2 For every 3 moles of hydrogen used, 2 moles of ammonia are made.
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Do not miss this

  • You can flip any mole ratio upside down.
  • Pick the version that cancels the unit you want to get rid of.
  • The substance you start with goes in the denominator. The substance you want goes in the numerator.
  • This is the same factor-label method you used in the Moles unit.

9.3 The Stoichiometry Roadmap: What to Do Step by Step

Many stoichiometry problems follow this path when you start with grams and need grams of a different substance. First identify what you are given and what you are asked to find. Then use only the steps you need.

Always check that the equation is balanced before using any mole ratio. If the coefficients are wrong, every later conversion will also be wrong.

If you are confused, slow down and label the path before you calculate. Most mistakes in stoichiometry happen before the arithmetic even starts.

Stoichiometry Roadmap A vertical roadmap showing conversions from Grams A to Moles A, across the mole ratio bridge to Moles B, and to Grams B. STOICHIOMETRY ROADMAP Grams-to-Grams Conversions Step-by-Step ! CRITICAL CHECKPOINT Verify your chemical equation is balanced first! Grams A Given Mass 1 ÷ by Molar Mass A Convert to moles Moles A Intermediate 2 × Mole Ratio (B/A) Coefficients from equation Moles B Target Base 3 × by Molar Mass B Convert to grams Grams B Final Answer The Factor-Label Chain A factor-label equation layout demonstrating the conversion from Grams A to Grams B. THE FACTOR-LABEL CHAIN g A × 1 mol A Molar Mass A × mol B mol A × Molar Mass B 1 mol B = g B PRO-TIP Map the path (g A → mol A → mol B → g B) before you calculate.
The roadmap shows the full grams-to-grams logic in one place. Convert the given grams to moles, cross the balanced-equation mole-ratio bridge, then convert the target moles back to grams only if the problem asks for mass.
  • Step 1 — Convert to moles: Divide the grams given by the molar mass of that substance. Now you have moles.
  • Step 2 — Use the mole ratio: Multiply by the stoichiometric factor from the balanced equation. This switches you from moles of the given substance to moles of the wanted substance.
  • Step 3 — Convert to grams: Multiply by the molar mass of the wanted substance. Now you have grams of your answer.

In the factor-label chain, MM means molar mass in g/mol, and the middle fraction is the mole ratio taken directly from the balanced equation.

If you start with moles, skip Step 1. If your answer should be in moles, stop after Step 2.

9.4 Limiting Reactant: Which Reactant Runs Out First?

What happens when you do not have exactly the right amounts of each reactant? One ingredient runs out first and stops the reaction. That ingredient is called the limiting reactant (or limiting reagent).

Start here if you keep mixing up limiting and excess reactants. The limiting reactant is not the one with the smaller starting mass. It is the one that can make less product once the balanced equation is taken seriously.

Sandwich analogy: You have 3 slices of bread and 5 slices of cheese. Each sandwich needs 2 bread + 1 cheese. You can only make 1 complete sandwich — bread runs out first. Bread is the limiting reactant. The extra cheese is the excess reactant.

In chemistry, the limiting reactant controls how much product you can make. Once it is used up, the reaction stops — even if other reactants are left over.

✗ Wrong comparison

Compare raw grams and pick the smaller number.

That fails because different substances have different molar masses and different coefficients.

✓ Correct comparison

Convert each reactant through the balanced equation and ask how much product each could make.

The reactant that makes less product is limiting.

How to Find the Limiting Reactant
For each reactant, calculate how much product it would make.
The reactant that produces less product is the limiting reactant.
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Common mistake

  • The instinct is to pick the reactant with fewer grams.
  • That is wrong — you must compare moles relative to the equation, not raw grams.
  • Always convert to moles first.

9.5 Theoretical Yield, Actual Yield, and Percent Yield

Once you know the limiting reactant, you know the maximum amount of product the reaction could make. That prediction is the theoretical yield. What you actually collect in the lab is usually smaller.

Here are the three terms and what each one refers to. They build on each other — you need the theoretical yield before you can calculate percent yield.

Theoretical yield The maximum amount of product the reaction could make, based on stoichiometry and the limiting reactant. Actual yield The amount of product you actually collect in the lab. Percent yield Percent yield compares what you collected to the maximum you could have collected.
Percent Yield Formula Percent Yield = (Actual Yield ÷ Theoretical Yield) × 100%

Actual yield goes on top because it is the amount you got. Theoretical yield goes on the bottom because it is the maximum predicted amount.

Example: If stoichiometry says you should make 10.0 g of product (theoretical yield), but you only collect 8.5 g in lab (actual yield), your percent yield = (8.5 ÷ 10.0) × 100% = 85%.
Limiting reactant and theoretical yield stoichiometry example A worked stoichiometry pathway showing two reactants compared in parallel to identify the limiting reactant and then convert to theoretical yield mass. LIMITING REACTANT & THEORETICAL YIELD STOICHIOMETRY STEP 1: INITIAL STOICHIOMETRIC BALANCED EQUATION 4 NH3 + 5 O2 → 4 NO + 6 H2O Given: 17.0 g | MM: 17.03 g/mol Given: 32.0 g | MM: 32.00 g/mol Target Product | MM: 30.01 g/mol STEP 2: PARALLEL REACTANT PATHWAY ANALYSIS Pathway A: NH3 Is it limiting? 1. Solve Initial Component Moles: 17.0 g NH3 × ( 1 mol / 17.03 g ) = 1.00 mol NH3 2. Extrapolate Yield Potential for Product (NO): 1.00 mol NH3 × ( 4 mol NO / 4 mol NH3 ) = 1.00 mol NO Max Available Yield Target: 1.00 mol NO Pathway B: O2 Is it limiting? 1. Solve Initial Component Moles: 32.0 g O2 × ( 1 mol / 32.00 g ) = 1.00 mol O2 2. Extrapolate Yield Potential for Product (NO): 1.00 mol O2 × ( 4 mol NO / 5 mol O2 ) = 0.80 mol NO Max Available Yield Target: 0.80 mol NO LIMITING REACTANT IDENTIFIED: O2 STEP 3: CONVERT STRUCTURAL LIMIT TO THEORETICAL YIELD MASS 0.80 mol NO × ( 30.01 g NO / 1 mol ) = 24.0 g NO (Theoretical Yield)
This yield map shows the full limiting-reactant logic in one picture. Convert each reactant to moles, ask how much product each one could make, choose the smaller product amount as the limiting path, then convert that product amount into the theoretical yield mass.
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If your answer is above 100%

  • True chemical yield should not exceed 100%, but a measured value above 100% can happen if the product is wet, impure, or measured incorrectly.
  • If your calculation gives more than 100%, something went wrong — recheck your math or your measurement.
✦ Practice Problems
Practice stoichiometry now, while mole ratios and limiting-reactant logic are still fresh.
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Next step after Unit 09

Stoichiometry is the main quantity tool for the rest of chemistry. Once these setups feel solid, move into chemical bonding for the next big content shift, and come back to stoichiometry whenever reaction quantities appear again. The best follow-up is the Unit 09 Practice page, the full practice hub, and Why Practice Tests Beat Rereading when you want stronger multi-step retrieval.

General Chemistry · Unit 09 · Stoichiometry