Grade 10-11 (age 15-17)

high school Chemical Equations and Stoichiometry

A chemical equation shows not only which substances react and what forms, but also how much of each takes part. The ratio of coefficients is the mole ratio, and for gases at the same temperature and pressure it is also the volume ratio. When one reactant runs out first the reaction stops; this limiting reagent sets how much product forms, and the other reactant is left over. The number of each kind of atom stays the same before and after, so the total mass is conserved. Changing the amounts of hydrogen and oxygen in the diagrams lets you check the mole ratio, which reactant runs out first and the conservation of mass in turn.

What a Chemical Equation Tells You

Moles of H₂ reacting2 mol

💡 Coefficients = Mole Ratio

①The coefficient ratio of 2H₂ + O₂ → 2H₂O is 2 : 1 : 2

②It is both a molecule ratio and a mole ratio, so 2 mol of H₂ reacts with 1 mol of O₂ to make 2 mol of H₂O

③Increasing H₂ makes all three bars grow together, always keeping 2 : 1 : 2

④In mass, 4 g of H₂ and 32 g of O₂ become 36 g of H₂O; the total moles drop from 3 mol to 2 mol, but the total mass stays the same

The Limiting Reagent

H₂ added4 mol

O₂ added1.5 mol

Finding the Limiting Reagent

Compare mol suppliedcoefficient → smallest is the limiting reagent

The reaction stops once the limiting reagent is used up, and the limiting reagent sets the amount of product

🔍 Checking the Starting Values

①For all 4 mol of H₂ to react, 4 ÷ 2 = 2 mol of O₂ is needed, but only 1.5 mol was added

②So O₂ runs out first and is the limiting reagent: 1.5 mol of O₂ reacts with 3 mol of H₂ to make 3 mol of H₂O

③The 1 mol of H₂ that cannot react is left over; a reactant left over like this is the excess reagent

④Raise O₂ to 2 mol and both reactants are used up exactly; add more than that and H₂ becomes the limiting reagent instead

Conservation of Mass

total mass of reactants = total mass of products

A reaction only rearranges atoms; it neither creates nor destroys them

Moles → Mass

m = n × M

n: amount in moles, M: molar mass (g/mol)

💡 Steps for Quantity Problems

①Balance the equation so that each element has the same number of atoms before and after

②Convert the given quantity (mass, gas volume or particle count) to moles

③Use the coefficient ratio to find the moles of the substance you want

④Convert those moles back to the unit the question asks for (mass, volume and so on)

Work It Out

Example 1

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

The coefficient ratio is the mole ratio, so write the ratio of H₂ to H₂O.

H₂ : H₂O = 2 : 2 = 1 : 1

With a 1 : 1 ratio, the moles of H₂ that react equal the moles of H₂O formed.

4 mol H₂ → 4 mol H₂O

▸ 4 mol

Set H₂ to 4 mol in the first diagram and the H₂O bar also reaches 4 mol; the O₂ needed is 2 mol.

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 of carbon to moles.

n(C) = 6 g ÷ 12 g/mol = 0.5 mol

Use the ratio C : CO₂ = 1 : 1 to find the moles of CO₂, then multiply by the molar mass M(CO₂) = 12 + 2 × 16 = 44 g/mol.

0.5 mol × 44 g/mol = 22 g

▸ 22 g

This follows the usual path mass → moles → coefficient ratio → moles → mass. Carbon at 6 g plus oxygen at 16 g gives 22 g of CO₂, so mass is conserved too.

Summary

The Core of Stoichiometry

coefficient ratio = mole ratio = molecule ratio = volume ratio (gases at the same temperature and pressure)

Every quantitative relationship starts from the coefficients of the chemical equation

2022 KICE mock Chemistry I type, adapted

In N₂ + 3H₂ → 2NH₃, 2 mol of N₂ and 3 mol of H₂ react. What is the most NH₃ that can form? (One reactant falls short.)

①1 mol

②2 mol

③3 mol

④4 mol

⑤6 mol

▸ ② 2 mol

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

Apply the ratio NH₃ : H₂ = 2 : 3 to the 3 mol of limiting H₂.

3 mol H₂ × 23 = 2 mol NH₃

🎯 Exam Points

①The coefficient ratio is the mole ratio, and for gases at the same temperature and pressure it is also the volume ratio

②The limiting reagent is the reactant with the smallest moles ÷ coefficient

③Total mass does not change in a reaction, but the total number of moles can

④Moles → mass: m = n M; moles of a gas at 0 °C and 1 atm → volume: V = 22.4 L × n

⑤Balancing: make the number of atoms of each element equal on both sides

← Previous

Electronegativity & Polarity

Oxidation-Reduction (Redox) Reaction

Last updated 2026-10-17

Was this helpful? Support seegongsik