Science Double Award Co-Ordinated (0654) iGCSE 2026 Chemistry 3 (Stoichiometry)

What is the molecular formula of water?

H₂O

What is the molecular formula of carbon dioxide?

CO₂

What is the molecular formula of methane?

CH₄

Define molecular formula.

The number and type of different atoms in one molecule of a compound.

Why do ionic compounds not have molecular formulas?

Because they form a giant lattice rather than individual molecules, so their formula represents the ratio of ions.

A compound contains 2 hydrogen atoms and 1 oxygen atom. What is its molecular formula?

H₂O

Deduce the formula of a compound with 1 magnesium ion (Mg²⁺) and 2 chloride ions (Cl⁻).

MgCl₂

Deduce the formula of a compound with 1 calcium ion (Ca²⁺) and 1 sulfate ion (SO₄²⁻).

CaSO₄

What is the relationship between ionic charge and the formula of an ionic compound?

The total positive and negative charges must balance to form a neutral compound.

A model shows 3 oxygen atoms for every 2 aluminum atoms. What is the formula?

Al₂O₃

A compound contains sodium (Na⁺) and phosphate (PO₄³⁻) ions. Deduce its formula.

Na₃PO₄

What is a word equation?

A way of representing a chemical reaction using the names of the reactants and products.

What is a symbol equation?

A way of representing a reaction using chemical formulas instead of names.

What are state symbols, and what do they represent?

(s) = solid, (l) = liquid, (g) = gas, (aq) = aqueous (dissolved in water).

Write the word equation for the reaction of hydrogen and oxygen to form water.

Hydrogen + Oxygen → Water

Write the symbol equation for the reaction of hydrogen and oxygen to form water.

2H₂(g) + O₂(g) → 2H₂O(l)

Why must chemical equations be balanced?

The law of conservation of mass states that atoms cannot be created or destroyed in a reaction.

What does (aq) mean in a chemical equation?

The substance is dissolved in water (aqueous solution).

Why do we include state symbols in equations?

To show the physical state of each substance and provide more information about the reaction.

Write the balanced symbol equation for the reaction of sodium and chlorine to form sodium chloride. Include state symbols.

2Na(s) + Cl₂(g) → 2NaCl(s)

Write the word equation for the reaction of calcium carbonate with hydrochloric acid.

Calcium carbonate + Hydrochloric acid → Calcium chloride + Carbon dioxide + Water

Write the balanced symbol equation with state symbols for the reaction of calcium carbonate and hydrochloric acid.

CaCO₃(s) + 2HCl(aq) → CaCl₂(aq) + CO₂(g) + H₂O(l)

Write the ionic equation for the neutralization of hydrochloric acid with sodium hydroxide.

H⁺(aq) + OH⁻(aq) → H₂O(l)

Compare a word equation and a symbol equation using an example.

Word equation: Magnesium + Oxygen → Magnesium oxide. Symbol equation: 2Mg(s) + O₂(g) → 2MgO(s)

Identify the reactants and products in the equation: Zn(s) + 2HCl(aq) → ZnCl₂(aq) + H₂(g).

Reactants: Zn, HCl. Products: ZnCl₂, H₂.

How can you tell if a reaction produces a gas by looking at the equation?

A gas will have the state symbol (g) in the products, e.g., CO₂(g), H₂(g), O₂(g).

Why is it necessary to balance equations before using them in calculations?

To ensure the correct mole ratio is used, which follows the law of conservation of mass.

Given the reaction aluminum reacts with oxygen to form aluminum oxide, deduce the balanced symbol equation with state symbols.

4Al(s) + 3O₂(g) → 2Al₂O₃(s)

A reaction produces barium sulfate as a precipitate. Write the ionic equation for this reaction.

Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s)

What is relative atomic mass (Aᵣ)?

The average mass of the isotopes of an element compared to 1/12th of the mass of a carbon-12 atom.

Why is relative atomic mass (Aᵣ) not always a whole number?

Because it is the weighted average of all naturally occurring isotopes of an element.

What is relative molecular mass (Mᵣ)?

The sum of the relative atomic masses of all atoms in a molecule.

What is relative formula mass (Mᵣ)?

The sum of the relative atomic masses of all atoms in an ionic compound.

What is the difference between relative molecular mass and relative formula mass?

Relative molecular mass is used for covalent molecules, while relative formula mass is used for ionic compounds.

How do you calculate the relative molecular mass (Mᵣ) of H₂O?

(2 × 1) + (1 × 16) = 18.

How do you calculate the relative formula mass (Mᵣ) of NaCl?

(1 × 23) + (1 × 35.5) = 58.5.

In what units can concentration be measured?

g/dm³ (grams per cubic decimetre) or mol/dm³ (moles per cubic decimetre).

What is the unit for the amount of substance?

The mole (mol).

How many particles are in one mole of a substance?

6.02 × 10²³ particles (Avogadro's constant).

What does Avogadro's constant represent?

The number of atoms, ions, or molecules in one mole of a substance.

What is the formula to calculate the amount of substance (mol)?

Amount of substance (mol) = Mass (g) / Molar mass (g/mol).

How do you calculate the mass of a substance?

Mass (g) = Amount of substance (mol) × Molar mass (g/mol).

How do you calculate the molar mass of a substance?

Molar mass (g/mol) = Mass (g) / Amount of substance (mol).

How do you calculate the relative atomic mass (Aᵣ) or relative molecular/formula mass (Mᵣ)?

By summing the relative atomic masses of the elements in a molecule or formula unit.

How do you calculate the number of particles in a sample?

Number of particles = Amount of substance (mol) × Avogadro's constant (6.02 × 10²³).

Calculate the amount of substance in 20 g of NaCl (Mᵣ = 58.5 g/mol).

Amount (mol) = 20 g ÷ 58.5 g/mol = 0.342 mol.

Calculate the mass of 2 moles of H₂O (Mᵣ = 18 g/mol).

Mass (g) = 2 mol × 18 g/mol = 36 g.

Calculate the molar mass of a substance if 5 moles have a mass of 200 g.

Molar mass (g/mol) = 200 g ÷ 5 mol = 40 g/mol.

Calculate the relative formula mass of CaCO₃. (Aᵣ: Ca = 40, C = 12, O = 16)

Mᵣ = (1 × 40) + (1 × 12) + (3 × 16) = 100 g/mol.

Calculate the number of molecules in 0.5 mol of CO₂. (Avogadro's constant = 6.02 × 10²³)

Number of molecules = 0.5 mol × (6.02 × 10²³) = 3.01 × 10²³ molecules.

Calculate the amount of substance in 100 cm³ (0.1 dm³) of a 2 mol/dm³ HCl solution.

Amount (mol) = Concentration (mol/dm³) × Volume (dm³) = 2 × 0.1 = 0.2 mol.

Calculate the mass of solute in 250 cm³ (0.25 dm³) of a 4 g/dm³ NaOH solution.

Mass (g) = Concentration (g/dm³) × Volume (dm³) = 4 × 0.25 = 1 g.

What is the molar gas volume at room temperature and pressure (r.t.p.)?

24 dm³ per mole of gas.

How do you calculate the volume of gas at r.t.p.?

Volume (dm³) = Amount (mol) × 24.

Calculate the volume of 2 moles of oxygen gas (O₂) at r.t.p.

Volume = 2 mol × 24 dm³/mol = 48 dm³.

How do you convert cm³ to dm³?

Divide by 1000 (1 dm³ = 1000 cm³).

Convert 250 cm³ to dm³.

250 cm³ ÷ 1000 = 0.25 dm³.

How do you calculate the amount of gas (mol) from volume at r.t.p.?

Amount (mol) = Volume (dm³) ÷ 24.

Calculate the number of moles in 72 dm³ of hydrogen gas (H₂) at r.t.p.

Amount = 72 ÷ 24 = 3 mol.

How do you calculate stoichiometric reacting masses?

Use the balanced chemical equation to find the molar ratio and convert between mass and moles.

In the reaction 2Mg + O₂ → 2MgO, how much MgO is produced from 12 g of Mg? (Mᵣ of Mg = 24, MgO = 40)

Moles of Mg = 12 ÷ 24 = 0.5 mol. MgO produced = 0.5 mol × 40 = 20 g.

How much volume of hydrogen gas is produced when 4 g of magnesium reacts with excess hydrochloric acid? (Equation: Mg + 2HCl → MgCl₂ + H₂, Aᵣ: Mg = 24, Molar gas volume = 24 dm³ at r.t.p.)

Moles of Mg = 4 ÷ 24 = 0.167 mol. From the equation, 1 mol Mg produces 1 mol H₂, so 0.167 mol Mg produces 0.167 mol H₂. Volume of H₂ = 0.167 × 24 = 4 dm³.

What volume of carbon dioxide gas is produced from the decomposition of 50 g of CaCO₃? (Equation: CaCO₃ → CaO + CO₂, Mᵣ: CaCO₃ = 100, Molar gas volume = 24 dm³ at r.t.p.)

Moles of CaCO₃ = 50 ÷ 100 = 0.5 mol. From the equation, 1 mol CaCO₃ produces 1 mol CO₂, so 0.5 mol produces 0.5 mol CO₂. Volume of CO₂ = 0.5 × 24 = 12 dm³.

Calculate the concentration in mol/dm³ of a solution containing 20 g of NaOH in 500 cm³ of solution. (Mᵣ: NaOH = 40)

Amount of NaOH = 20 ÷ 40 = 0.5 mol. Convert cm³ to dm³: 500 ÷ 1000 = 0.5 dm³. Concentration = 0.5 mol ÷ 0.5 dm³ = 1 mol/dm³.

What mass of H₂SO₄ is needed to prepare 250 cm³ of a 2 mol/dm³ solution? (Mᵣ: H₂SO₄ = 98)

Convert cm³ to dm³: 250 ÷ 1000 = 0.25 dm³. Moles of H₂SO₄ = 2 × 0.25 = 0.5 mol. Mass = 0.5 × 98 = 49 g.

Determine the limiting reagent when 10 g of H₂ reacts with 80 g of O₂. (Equation: 2H₂ + O₂ → 2H₂O, Aᵣ: H = 1, O = 16)

Moles of H₂ = 10 ÷ (2 × 1) = 5 mol. Moles of O₂ = 80 ÷ (16 × 2) = 2.5 mol. From the equation, 2 mol H₂ reacts with 1 mol O₂, so 5 mol H₂ requires 2.5 mol O₂. Since both are in the correct ratio, neither is limiting. Both react completely.

Determine the limiting reagent when 20 g of Mg reacts with 40 g of HCl. (Equation: Mg + 2HCl → MgCl₂ + H₂, Aᵣ: Mg = 24, HCl = 36.5)

Moles of Mg = 20 ÷ 24 = 0.833 mol. Moles of HCl = 40 ÷ 36.5 = 1.096 mol. From the equation, 1 mol Mg reacts with 2 mol HCl, so 0.833 mol Mg requires 1.67 mol HCl. Since there is only 1.096 mol HCl available, HCl is the limiting reagent.