Chemical Equations and Stoichiometry

Stoichiometry

Coefficients in a chemical equation are mole ratios, and for gases they are volume ratios too. When one reactant runs out first, that limiting reagent sets how much product forms, while total mass is conserved. You convert given amounts to moles, apply the coefficient ratio, then convert back to the unit you need. Here you change H₂ and O₂ supplied to see mole ratios, the limiting reagent, and mass conservation on the sliders.

What a Chemical Equation Tells You

H₂ moles2 mol

💡 Key: Coefficient = Mole Ratio

①In 2H₂ + O₂ → 2H₂O, coefficient ratio = 2:1:2

②That is the mole ratio = 2:1:2

③2 mol H₂ needs 1 mol O₂

④Exactly 2 mol H₂O is produced

Limiting Reagent

H₂ supplied4 mol

O₂ supplied1.5 mol

How to Find the Limiting Reagent

Compare mol suppliedcoefficient → smallest is the limiting reagent

The limiting reagent determines the amount of product

🔍 Try the Sliders!

①With H₂ = 4 mol, O₂ = 1.5 mol: O₂ is limiting

②O₂ 1.5 mol → 3 mol H₂O → 1 mol H₂ left

③Reaction ends when limiting reagent is gone

④Leftover reactant = excess

Conservation of Mass

Mass Conservation

total reactant mass = total product mass

Atoms are neither created nor destroyed

Mole → Mass

m = n × M

n: moles, M: molar mass [g/mol]

💡 Order of Stoichiometric Calculations

①Balance the equation (method of undetermined coefficients)

②Convert given quantity → moles

③Apply coefficient ratio → target moles

④Convert moles → required units (mass, volume, etc.)

Work It Out

Example 1

In 2H₂ + O₂ → 2H₂O, how many mol of H₂O form when 4 mol of H₂ react completely?

Coefficients are the mole ratio, so write the H₂-to-H₂O ratio.

H₂ : H₂O = 2 : 2 = 1 : 1

The 1:1 ratio means the moles of H₂O equal the moles of H₂.

4 mol H₂ → 4 mol H₂O

▸ 4 mol

The coefficient ratio 2:2 reduces to 1:1, so H₂ reacted equals H₂O produced.

Example 2

In C + O₂ → CO₂, what mass of CO₂ forms when 6 g of carbon (C) burns completely? (C=12, O=16)

Convert the mass to moles.

n(C) = 6 g ÷ 12 g/mol = 0.5 mol

Apply the 1:1 ratio C:CO₂ to get moles of CO₂, then convert to mass. (M(CO₂)=44)

0.5 mol × 44 g/mol = 22 g

▸ 22 g

Follow the standard chain: mass → mole → coefficient ratio → mole → mass.

Summary

Key of Stoichiometry

coefficient ratio = mole ratio = volume ratio (gases)

Coefficients in the equation are the key to all stoichiometric relations

2022 KICE mock Chemistry I type, adapted

In N₂ + 3H₂ → 2NH₃, reacting 2 mol of N₂ with 3 mol of H₂, what is the maximum amount of NH₃ formed? (one reactant runs short)

①1 mol

②2 mol

③3 mol

④4 mol

⑤6 mol

▸ ② 2 mol

Using all 2 mol of N₂ would need 6 mol of H₂, but only 3 mol of H₂ is available, so H₂ is the limiting reagent.

Apply the coefficient ratio 2 NH₃ / 3 H₂ to the 3 mol of limiting H₂.

3 mol H₂ × 23 = 2 mol NH₃

🎯 Exam Points

①Coefficient ratio = mole ratio (= volume ratio for gases)

②Limiting reagent: compare mol/coeff for each reactant

③Mass conservation: total mass before = total mass after

④Mole → mass: m = nM; mole → volume(STP): V = 22.4n

⑤Balancing: match atom counts of each element

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Electronegativity & Polarity

Redox Reactions

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