Cambridge IGCSE Chemistry 0620 · Extended · Syllabus 2026-2028

The Mole and the Avogadro Constant Worked ExamplesIGCSE Chemistry 0620

Exam-style mole calculations: the Avogadro constant and counting atoms and ions, concentration in g/dm3 and mol/dm3, amount, mass and molar mass, gas volumes at r.t.p. with a gas syringe, limiting reactants from a graph, titration results from burette readings, empirical and molecular formulae from percentages and from a crucible experiment, percentage purity and percentage yield, and a thermite limiting-reactant problem.

This page has 12 exam-style IGCSE Chemistry 0620 the mole and the avogadro constant worked examples — 2 Core, 8 Extended and 2 Challenge, 5 with diagrams. Each one shows the full method step by step, the final answer in the form a marker expects, and an examiner tip on where marks are commonly lost.

Example 1CoreState the Avogadro constant; concentration in g/dm3 and mol/dm3 with cm3 to dm3 conversion[4 marks]

The mole and concentration in two units

(a) State the number of particles in one mole of a substance. [1]

(b) $5.05$ g of potassium nitrate, $\text{KNO}_3$ , is dissolved in water to make $250\text{ cm}^3$ of solution.

Calculate the concentration of the solution in

(i) $\text{g/dm}^3$ and

(ii) $\text{mol/dm}^3$ . [3]

Example 2CorePaper 2 MCQ: number of atoms in a given amount of a molecular substance[1 mark]

Counting atoms, not molecules (multiple choice)

The Avogadro constant is $6.02 \times 10^{23}$ per mole.

How many atoms are there in $0.50$ mol of sulfur dioxide, $\text{SO}_2$ ?

A $3.01 \times 10^{23}$ B $6.02 \times 10^{23}$ C $9.03 \times 10^{23}$ D $1.81 \times 10^{24}$

Example 3ExtendedUse amount = mass / molar mass for mass, molar mass and Ar; number of ions[5 marks]

Mass, amount and molar mass — identifying a metal

(a) Calculate the mass of $0.0250$ mol of sodium carbonate, $\text{Na}_2\text{CO}_3$ . [1]

(b) $16.65$ g of the chloride of a Group II metal, $\text{MCl}_2$ , contains $0.150$ mol of $\text{MCl}_2$ .

(i) Calculate the molar mass of $\text{MCl}_2$ . [1]

(ii) Identify metal M. [1]

(iii) Calculate the number of chloride ions in the $16.65$ g sample. [2]

Example 4ExtendedGas volume at r.t.p. → moles → mole ratio → mass of product[3 marks]

From a gas volume to a mass of product

Iron burns in chlorine to form iron(III) chloride.

$2\text{Fe(s)} + 3\text{Cl}_2\text{(g)} \rightarrow 2\text{FeCl}_3\text{(s)}$

$1.44\text{ dm}^3$ of chlorine, measured at r.t.p., reacts with an excess of iron.

Calculate the mass of iron(III) chloride formed.

Example 5ExtendedMoles from concentration and volume → ratio → gas volume in cm3; practical source of error[5 marks]

Gas volume from a solution, collected in a gas syringe

A student adds $20.0\text{ cm}^3$ of $0.150\text{ mol/dm}^3$ hydrochloric acid to an excess of marble chips (calcium carbonate) and collects the carbon dioxide in a gas syringe.

$\text{CaCO}_3\text{(s)} + 2\text{HCl(aq)} \rightarrow \text{CaCl}_2\text{(aq)} + \text{H}_2\text{O(l)} + \text{CO}_2\text{(g)}$

(a) Calculate the volume of carbon dioxide, in $\text{cm}^3$ at r.t.p., that should form. [3]

(b) The final syringe reading is shown. State the reading, and explain why adding more marble chips would not increase it. [1]

(c) Suggest one reason why a student repeating the experiment might record a smaller volume. [1]

NOT TO SCALE

Example 6ExtendedIdentify the limiting reactant; maximum gas volume and the mass at which the graph levels off[5 marks]

Limiting reactant from a gas-volume graph

Different masses of calcium carbonate were added, one at a time, to separate $25.0\text{ cm}^3$ portions of $1.00\text{ mol/dm}^3$ nitric acid. The total volume of carbon dioxide was measured at r.t.p.

$\text{CaCO}_3\text{(s)} + 2\text{HNO}_3\text{(aq)} \rightarrow \text{Ca(NO}_3)_2\text{(aq)} + \text{H}_2\text{O(l)} + \text{CO}_2\text{(g)}$

(a) Explain why the graph levels off. [1]

(b) Calculate the maximum volume of carbon dioxide that $25.0\text{ cm}^3$ of the acid can produce. [2]

(c) Calculate the mass of calcium carbonate that reacts exactly with $25.0\text{ cm}^3$ of the acid, and check your answer against the graph. [2]

Example 7ExtendedRead a burette, choose concordant titres, mean titre, then 1 : 2 mole ratio to find concentration[6 marks]

Titration: burette readings to concentration

$25.0\text{ cm}^3$ of aqueous sodium hydroxide was titrated with $0.100\text{ mol/dm}^3$ sulfuric acid, using methyl orange indicator.

$\text{H}_2\text{SO}_4\text{(aq)} + 2\text{NaOH(aq)} \rightarrow \text{Na}_2\text{SO}_4\text{(aq)} + 2\text{H}_2\text{O(l)}$

	rough	1	3
final reading / $\text{cm}^3$	12.90	12.60	25.50
initial reading / $\text{cm}^3$	0.00	0.30	12.90
volume of acid / $\text{cm}^3$	12.90	12.30	12.60

(a) The diagrams show the burette readings for titration 2. Complete the table. [2]

(b) Calculate the mean titre using the concordant results (within $0.10\text{ cm}^3$ of each other). [1]

(c) Calculate the concentration of the sodium hydroxide in $\text{mol/dm}^3$ . [3]

NOT TO SCALE

Example 8ExtendedEmpirical formula from percentage composition needing a ×2 step; molecular formula from Mr[4 marks]

Empirical and molecular formula from percentages

Compound Q contains $49.3\%$ carbon, $6.8\%$ hydrogen and $43.8\%$ oxygen by mass. Its relative molecular mass is 146.

(a) Calculate the empirical formula of Q. [3]

(b) Deduce the molecular formula of Q. [1]

Example 9ExtendedPractical: masses from a crucible experiment → empirical formula; procedure and sources of error[5 marks]

Empirical formula of an oxide from a crucible experiment

A student heated tin in a crucible, as shown, until it had all reacted with oxygen from the air.

	mass / g
empty crucible and lid	24.52
crucible, lid and tin	25.71
crucible, lid and tin oxide after heating to constant mass	26.03

(a) Calculate the empirical formula of the tin oxide. [3]

(b) Explain why the student lifted the lid slightly from time to time during heating. [1]

(c) State what heating to constant mass means and why it is done. [1]

NOT TO SCALE

Example 10ExtendedPercentage purity from mass loss of carbon dioxide; percentage yield of a product[5 marks]

Percentage purity and percentage yield

A $2.50$ g sample of limestone is added to excess hydrochloric acid in a flask on a balance. The impurities do not react.

$\text{CaCO}_3\text{(s)} + 2\text{HCl(aq)} \rightarrow \text{CaCl}_2\text{(aq)} + \text{H}_2\text{O(l)} + \text{CO}_2\text{(g)}$

The total mass decreases by $0.990$ g.

(a) Explain why the mass decreases, and state the purpose of the cotton wool. [2]

(b) Calculate the percentage purity of the calcium carbonate in the limestone. [2]

(c) Calcium chloride crystals are then made from the solution. The student obtains $2.10$ g. Calculate the percentage yield. [1]

NOT TO SCALE

Example 11ChallengeMoles from mass → 1 : 2 ratio → volume of solution in cm3; concentration in g/dm3[4 marks]

Volume of alkali to neutralise a diprotic acid

Ethanedioic acid, $\text{H}_2\text{C}_2\text{O}_4$ , reacts with sodium hydroxide:

$\text{H}_2\text{C}_2\text{O}_4\text{(aq)} + 2\text{NaOH(aq)} \rightarrow \text{Na}_2\text{C}_2\text{O}_4\text{(aq)} + 2\text{H}_2\text{O(l)}$

(a) $1.80$ g of ethanedioic acid is dissolved in water. What volume of $0.160\text{ mol/dm}^3$ aqueous sodium hydroxide exactly neutralises it? Give your answer in $\text{cm}^3$ . [3]

(b) State the concentration of the sodium hydroxide solution in $\text{g/dm}^3$ . [1]

Example 12ChallengeLimiting reactant from two masses; mass of product; excess remaining; percentage yield[6 marks]

Thermite: limiting reactant and percentage yield

A mixture of $8.00$ g of iron(III) oxide and $3.00$ g of aluminium powder is ignited.

$\text{Fe}_2\text{O}_3\text{(s)} + 2\text{Al(s)} \rightarrow \text{Al}_2\text{O}_3\text{(s)} + 2\text{Fe(l)}$

(a) Show which reactant is limiting. [2]

(b) Calculate the maximum mass of iron that can form. [1]

(c) Calculate the mass of the reactant in excess that is left over. [1]

(d) $4.90$ g of iron is collected. Calculate the percentage yield. [2]

These worked examples are written by Tutopiya to cover the same syllabus content as the official Cambridge IGCSE 0620 specification (2026-2028). They are exam-style questions, not reproductions of past paper questions.

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← Back to The Mole and the Avogadro Constant study notes

The Mole and the Avogadro Constant Worked ExamplesIGCSE Chemistry 0620

Example 1CoreState the Avogadro constant; concentration in g/dm3 and mol/dm3 with cm3 to dm3 conversion[4 marks]

The mole and concentration in two units

(a) State the number of particles in one mole of a substance. [1]

(b) $5.05$ g of potassium nitrate, $\text{KNO}_3$ , is dissolved in water to make $250\text{ cm}^3$ of solution.

Calculate the concentration of the solution in

(i) $\text{g/dm}^3$ and

(ii) $\text{mol/dm}^3$ . [3]

Example 2CorePaper 2 MCQ: number of atoms in a given amount of a molecular substance[1 mark]

Counting atoms, not molecules (multiple choice)

The Avogadro constant is $6.02 \times 10^{23}$ per mole.

How many atoms are there in $0.50$ mol of sulfur dioxide, $\text{SO}_2$ ?

A $3.01 \times 10^{23}$ B $6.02 \times 10^{23}$ C $9.03 \times 10^{23}$ D $1.81 \times 10^{24}$

Example 3ExtendedUse amount = mass / molar mass for mass, molar mass and Ar; number of ions[5 marks]

Mass, amount and molar mass — identifying a metal

(a) Calculate the mass of $0.0250$ mol of sodium carbonate, $\text{Na}_2\text{CO}_3$ . [1]

(b) $16.65$ g of the chloride of a Group II metal, $\text{MCl}_2$ , contains $0.150$ mol of $\text{MCl}_2$ .

(i) Calculate the molar mass of $\text{MCl}_2$ . [1]

(ii) Identify metal M. [1]

(iii) Calculate the number of chloride ions in the $16.65$ g sample. [2]

Example 4ExtendedGas volume at r.t.p. → moles → mole ratio → mass of product[3 marks]

From a gas volume to a mass of product

Iron burns in chlorine to form iron(III) chloride.

$2\text{Fe(s)} + 3\text{Cl}_2\text{(g)} \rightarrow 2\text{FeCl}_3\text{(s)}$

$1.44\text{ dm}^3$ of chlorine, measured at r.t.p., reacts with an excess of iron.

Calculate the mass of iron(III) chloride formed.

Example 5ExtendedMoles from concentration and volume → ratio → gas volume in cm3; practical source of error[5 marks]

Gas volume from a solution, collected in a gas syringe

A student adds $20.0\text{ cm}^3$ of $0.150\text{ mol/dm}^3$ hydrochloric acid to an excess of marble chips (calcium carbonate) and collects the carbon dioxide in a gas syringe.

$\text{CaCO}_3\text{(s)} + 2\text{HCl(aq)} \rightarrow \text{CaCl}_2\text{(aq)} + \text{H}_2\text{O(l)} + \text{CO}_2\text{(g)}$

(a) Calculate the volume of carbon dioxide, in $\text{cm}^3$ at r.t.p., that should form. [3]

(b) The final syringe reading is shown. State the reading, and explain why adding more marble chips would not increase it. [1]

(c) Suggest one reason why a student repeating the experiment might record a smaller volume. [1]

NOT TO SCALE

Example 6ExtendedIdentify the limiting reactant; maximum gas volume and the mass at which the graph levels off[5 marks]

Limiting reactant from a gas-volume graph

$\text{CaCO}_3\text{(s)} + 2\text{HNO}_3\text{(aq)} \rightarrow \text{Ca(NO}_3)_2\text{(aq)} + \text{H}_2\text{O(l)} + \text{CO}_2\text{(g)}$

(a) Explain why the graph levels off. [1]

(b) Calculate the maximum volume of carbon dioxide that $25.0\text{ cm}^3$ of the acid can produce. [2]

(c) Calculate the mass of calcium carbonate that reacts exactly with $25.0\text{ cm}^3$ of the acid, and check your answer against the graph. [2]

Example 7ExtendedRead a burette, choose concordant titres, mean titre, then 1 : 2 mole ratio to find concentration[6 marks]

Titration: burette readings to concentration

$25.0\text{ cm}^3$ of aqueous sodium hydroxide was titrated with $0.100\text{ mol/dm}^3$ sulfuric acid, using methyl orange indicator.

$\text{H}_2\text{SO}_4\text{(aq)} + 2\text{NaOH(aq)} \rightarrow \text{Na}_2\text{SO}_4\text{(aq)} + 2\text{H}_2\text{O(l)}$

	rough	1	3
final reading / $\text{cm}^3$	12.90	12.60	25.50
initial reading / $\text{cm}^3$	0.00	0.30	12.90
volume of acid / $\text{cm}^3$	12.90	12.30	12.60

(a) The diagrams show the burette readings for titration 2. Complete the table. [2]

(b) Calculate the mean titre using the concordant results (within $0.10\text{ cm}^3$ of each other). [1]

(c) Calculate the concentration of the sodium hydroxide in $\text{mol/dm}^3$ . [3]

NOT TO SCALE

Example 8ExtendedEmpirical formula from percentage composition needing a ×2 step; molecular formula from Mr[4 marks]

Empirical and molecular formula from percentages

Compound Q contains $49.3\%$ carbon, $6.8\%$ hydrogen and $43.8\%$ oxygen by mass. Its relative molecular mass is 146.

(a) Calculate the empirical formula of Q. [3]

(b) Deduce the molecular formula of Q. [1]

Example 9ExtendedPractical: masses from a crucible experiment → empirical formula; procedure and sources of error[5 marks]

Empirical formula of an oxide from a crucible experiment

A student heated tin in a crucible, as shown, until it had all reacted with oxygen from the air.

	mass / g
empty crucible and lid	24.52
crucible, lid and tin	25.71
crucible, lid and tin oxide after heating to constant mass	26.03

(a) Calculate the empirical formula of the tin oxide. [3]

(b) Explain why the student lifted the lid slightly from time to time during heating. [1]

(c) State what heating to constant mass means and why it is done. [1]

NOT TO SCALE

Example 10ExtendedPercentage purity from mass loss of carbon dioxide; percentage yield of a product[5 marks]

Percentage purity and percentage yield

A $2.50$ g sample of limestone is added to excess hydrochloric acid in a flask on a balance. The impurities do not react.

$\text{CaCO}_3\text{(s)} + 2\text{HCl(aq)} \rightarrow \text{CaCl}_2\text{(aq)} + \text{H}_2\text{O(l)} + \text{CO}_2\text{(g)}$

The total mass decreases by $0.990$ g.

(a) Explain why the mass decreases, and state the purpose of the cotton wool. [2]

(b) Calculate the percentage purity of the calcium carbonate in the limestone. [2]

(c) Calcium chloride crystals are then made from the solution. The student obtains $2.10$ g. Calculate the percentage yield. [1]

NOT TO SCALE

Example 11ChallengeMoles from mass → 1 : 2 ratio → volume of solution in cm3; concentration in g/dm3[4 marks]

Volume of alkali to neutralise a diprotic acid

Ethanedioic acid, $\text{H}_2\text{C}_2\text{O}_4$ , reacts with sodium hydroxide:

$\text{H}_2\text{C}_2\text{O}_4\text{(aq)} + 2\text{NaOH(aq)} \rightarrow \text{Na}_2\text{C}_2\text{O}_4\text{(aq)} + 2\text{H}_2\text{O(l)}$

(a) $1.80$ g of ethanedioic acid is dissolved in water. What volume of $0.160\text{ mol/dm}^3$ aqueous sodium hydroxide exactly neutralises it? Give your answer in $\text{cm}^3$ . [3]

(b) State the concentration of the sodium hydroxide solution in $\text{g/dm}^3$ . [1]

Example 12ChallengeLimiting reactant from two masses; mass of product; excess remaining; percentage yield[6 marks]

Thermite: limiting reactant and percentage yield

A mixture of $8.00$ g of iron(III) oxide and $3.00$ g of aluminium powder is ignited.

$\text{Fe}_2\text{O}_3\text{(s)} + 2\text{Al(s)} \rightarrow \text{Al}_2\text{O}_3\text{(s)} + 2\text{Fe(l)}$

(a) Show which reactant is limiting. [2]

(b) Calculate the maximum mass of iron that can form. [1]

(c) Calculate the mass of the reactant in excess that is left over. [1]

(d) $4.90$ g of iron is collected. Calculate the percentage yield. [2]

The Mole and the Avogadro Constant Worked ExamplesIGCSE Chemistry 0620

The mole and concentration in two units

Counting atoms, not molecules (multiple choice)

Mass, amount and molar mass — identifying a metal

From a gas volume to a mass of product

Gas volume from a solution, collected in a gas syringe

Limiting reactant from a gas-volume graph

Titration: burette readings to concentration

Empirical and molecular formula from percentages

Empirical formula of an oxide from a crucible experiment

Percentage purity and percentage yield

Volume of alkali to neutralise a diprotic acid

Thermite: limiting reactant and percentage yield

More IGCSE Chemistry 0620 worked examples

Stoichiometry

States of Matter

Atoms, Elements and Compounds

Electrochemistry

Chemical Energetics

Chemical Reactions

Acids, Bases and Salts

The Periodic Table

Metals

Chemistry of the Environment

Organic Chemistry

Experimental Techniques and Chemical Analysis

Alternative to Practical

The Mole and the Avogadro Constant Worked ExamplesIGCSE Chemistry 0620

The mole and concentration in two units

Counting atoms, not molecules (multiple choice)

Mass, amount and molar mass — identifying a metal

From a gas volume to a mass of product

Gas volume from a solution, collected in a gas syringe

Limiting reactant from a gas-volume graph

Titration: burette readings to concentration

Empirical and molecular formula from percentages

Empirical formula of an oxide from a crucible experiment

Percentage purity and percentage yield

Volume of alkali to neutralise a diprotic acid

Thermite: limiting reactant and percentage yield

More IGCSE Chemistry 0620 worked examples

Stoichiometry

States of Matter

Atoms, Elements and Compounds

Electrochemistry

Chemical Energetics

Chemical Reactions

Acids, Bases and Salts

The Periodic Table

Metals

Chemistry of the Environment

Organic Chemistry

Experimental Techniques and Chemical Analysis

Alternative to Practical