Cambridge IGCSE Chemistry 0620 · Extended · Syllabus 2026-2028

Fertilisers Worked ExamplesIGCSE Chemistry 0620

Exam-style fertiliser questions: which compounds supply nitrogen, phosphorus and potassium in NPK fertilisers, ammonium salts and nitrates, percentage of nitrogen by mass, making ammonium sulfate by titration, why ammonium fertilisers lose ammonia when mixed with bases, identifying a fertiliser from ion tests, the Haber process as the source of ammonia, gas-volume, percentage-yield, purity, empirical-formula and limiting-reactant calculations, and the effect of fertiliser run-off on dissolved oxygen in rivers.

This page has 12 exam-style IGCSE Chemistry 0620 fertilisers worked examples — 2 Core, 8 Extended and 2 Challenge, 6 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 1CorePaper 2 MCQ: identify from formulae a mixture that supplies N, P and K[1 mark]

Which mixture supplies nitrogen, phosphorus and potassium? (multiple choice)

A gardener wants a fertiliser that provides all three of the elements nitrogen, phosphorus and potassium.

Which mixture should be used?

A $\text{NH}_4\text{NO}_3$ and KCl

B $\text{(NH}_4\text{)}_2\text{SO}_4$ and $\text{K}_3\text{PO}_4$

C $\text{KNO}_3$ and $\text{NH}_4\text{Cl}$

D $\text{(NH}_4\text{)}_3\text{PO}_4$ and NaCl

Example 2CoreState that ammonium salts and nitrates are fertilisers; which compound supplies N, P and K[4 marks]

Reading an NPK fertiliser label

The label shows the compounds in a bag of NPK fertiliser.

(a) Name the compound on the label that supplies

(i) phosphorus

(ii) potassium. [2]

(b) Ammonium salts and nitrates are both used as fertilisers. Name the compound on the label that is both an ammonium salt and a nitrate. [1]

(c) State why farmers add NPK fertilisers to soil. [1]

Example 3ExtendedPercentage composition by mass; mass of an element in a given mass of compound[5 marks]

Comparing the percentage of nitrogen in three fertilisers

Three compounds used in fertilisers are ammonium sulfate, $\text{(NH}_4\text{)}_2\text{SO}_4$ , ammonium phosphate, $\text{(NH}_4\text{)}_3\text{PO}_4$ , and potassium nitrate, $\text{KNO}_3$ .

(a) Calculate the percentage by mass of nitrogen in each compound. [3]

(b) Calculate the mass of nitrogen in a $50$ kg bag of pure ammonium sulfate. [1]

(c) A farmer needs a fertiliser that supplies nitrogen and potassium. Suggest which of the three compounds to use, and give a reason. [1]

$A_r$ : H = 1; N = 14; O = 16; P = 31; S = 32; K = 39

Example 4ExtendedPaper 6/4: read burette, titre, titration calculation with 2:1 ratio; obtain crystals of the salt[7 marks]

Making ammonium sulfate by titration: burette readings and concentration

A student places $25.0 \text{ cm}^3$ of aqueous ammonia in a conical flask with a few drops of methyl orange, then adds $0.100 \text{ mol/dm}^3$ sulfuric acid from a burette until the end point. The diagram shows the burette at the start and at the end point.

The equation is $2\text{NH}_3 + \text{H}_2\text{SO}_4 \rightarrow \text{(NH}_4\text{)}_2\text{SO}_4$ .

(a) Record both burette readings and calculate the volume of acid added. [2]

(b) Calculate the concentration of the aqueous ammonia in $\text{mol/dm}^3$ . [3]

(c) Describe how the student could use this result to obtain pure, dry crystals of ammonium sulfate. [2]

1
(a) Read the bottom of the meniscus, to $0.05 \text{ cm}^3$ : start $= 1.20 \text{ cm}^3$ , end $= 22.80 \text{ cm}^3$ .
2
Volume of acid added = final − initial reading.
$22.80 - 1.20 = 21.60 \text{ cm}^3$
3
(b) Moles of sulfuric acid (convert $\text{cm}^3$ to $\text{dm}^3$ by $\div 1000$ ):
$n(\text{H}_2\text{SO}_4) = 0.100 \times \frac{21.60}{1000} = 2.16 \times 10^{-3} \text{ mol}$
4
Use the mole ratio $\text{NH}_3 : \text{H}_2\text{SO}_4 = 2 : 1$ .
$n(\text{NH}_3) = 2 \times 2.16 \times 10^{-3} = 4.32 \times 10^{-3} \text{ mol}$
5
Divide by the volume of ammonia in $\text{dm}^3$ .
$c = \frac{4.32 \times 10^{-3}}{0.0250} = 0.173 \text{ mol/dm}^3$
6
(c) Repeat the titration with $25.0 \text{ cm}^3$ of ammonia and $21.60 \text{ cm}^3$ of acid but without the indicator (which would contaminate the salt). Heat the solution to evaporate some water until the crystallisation point, cool to form crystals, then filter and dry them (between filter papers).

Answer

(a) $1.20 \text{ cm}^3$ and $22.80 \text{ cm}^3$ ; $21.60 \text{ cm}^3$ (b) $0.173 \text{ mol/dm}^3$ (c) Repeat without indicator using the same volumes; evaporate to crystallisation point, cool, filter, dry

Examiner tip

The June 2025 examiner report stated that "Readings recorded from a given item of apparatus should all be recorded to the same resolution" — record burette readings to two decimal places, e.g. $1.20$ not $1.2$ . In (b), a common slip is to forget the 2 : 1 ratio and give $0.0864 \text{ mol/dm}^3$ .

Example 5ExtendedBases react with ammonium salts releasing ammonia; test for ammonia; equation and ionic equation[6 marks]

Why ammonium fertilisers should not be mixed with lime

A farmer spreads calcium hydroxide (slaked lime) on a field at the same time as a fertiliser F. To investigate, a student warms F with calcium hydroxide as shown. The damp red litmus paper turns blue.

(a) Name the gas that turns the litmus blue and the type of ion that fertiliser F must contain. [2]

(b) F is ammonium sulfate. Write the balanced equation for its reaction with calcium hydroxide. [2]

(c) Write the ionic equation for the reaction that produces the gas. [1]

(d) Explain why the farmer should not spread the two substances at the same time. [1]

NOT TO SCALE

Example 6ExtendedPaper 2 MCQ: qualitative analysis of a fertiliser (ammonium, sulfate, chloride tests)[1 mark]

Identifying a fertiliser from ion tests (multiple choice)

A solution of fertiliser G is tested.

Warmed with aqueous sodium hydroxide: a gas turns damp red litmus paper blue.
Acidified with dilute nitric acid, then aqueous barium nitrate added: white precipitate.
Acidified with dilute nitric acid, then aqueous silver nitrate added: no precipitate.

What is G?

A ammonium chloride

B ammonium sulfate

C potassium sulfate

D ammonium nitrate

Example 7ExtendedHaber process: sources, conditions, equation; neutralisation to make a nitrate fertiliser[8 marks]

From air and methane to ammonium nitrate: the Haber process link

The flow chart shows how fertiliser T is made.

(a) Name gas P, gas Q and raw material M. [3]

(b) State the typical temperature, pressure and catalyst used in the reactor. [2]

(c) Write the symbol equation, with state symbols, for the reaction in the reactor. [2]

(d) Product R reacts with acid S to form fertiliser T, which supplies nitrogen only. Name S and T. [1]

Example 8ExtendedGas volume (cm³) → moles → mass of product; percentage yield[5 marks]

Ammonia gas to ammonium nitrate: mass and percentage yield

Ammonia gas is passed into nitric acid: $\text{NH}_3\text{(g)} + \text{HNO}_3\text{(aq)} \rightarrow \text{NH}_4\text{NO}_3\text{(aq)}$

(a) $1800 \text{ cm}^3$ of ammonia, measured at r.t.p., reacts completely. Calculate the maximum mass of ammonium nitrate that can form. [3]

(b) The student obtains $5.10$ g of dry ammonium nitrate crystals. Calculate the percentage yield. [1]

(c) Suggest one reason why the yield is less than $100\%$ . [1]

$A_r$ : H = 1; N = 14; O = 16. The volume of one mole of any gas at r.t.p. is $24 \text{ dm}^3$ .

Example 9ExtendedPractical: gas syringe reading → moles of ammonia → mass of salt → percentage purity; sources of error[7 marks]

Purity of an ammonium sulfate fertiliser from a gas syringe

A $0.250$ g sample of impure ammonium sulfate fertiliser is warmed with excess aqueous sodium hydroxide. The ammonia produced is collected in a gas syringe, as shown. The impurities do not produce a gas.

$\text{(NH}_4\text{)}_2\text{SO}_4\text{(s)} + 2\text{NaOH}\text{(aq)} \rightarrow \text{Na}_2\text{SO}_4\text{(aq)} + 2\text{NH}_3\text{(g)} + 2\text{H}_2\text{O}\text{(l)}$

(a) Read the volume of ammonia collected. [1]

(b) Calculate the mass of ammonium sulfate in the sample, and hence its percentage purity. The volume is measured at r.t.p. [4]

(c) Ammonia is very soluble in water. Explain how this affects the calculated purity, and suggest one improvement. [2]

$A_r$ : H = 1; N = 14; O = 16; S = 32

NOT TO SCALE

Example 10ExtendedRead a graph; percentage decrease; nitrates/phosphates cause deoxygenation; suggest recovery[6 marks]

Fertiliser run-off and dissolved oxygen in a river

A stream that drains farmland joins a river at distance $0$ km. The graph shows the dissolved oxygen in the river at different distances.

(a) State the dissolved oxygen concentration upstream of the stream, and the lowest value in the river. [2]

(b) Calculate the percentage decrease from the upstream value to the lowest value. [1]

(c) Name two substances from fertilisers that could cause this decrease, and state the harm caused. [2]

(d) Suggest why the dissolved oxygen increases again further downstream. [1]

Example 11ChallengeEmpirical formula from percentage composition; molecular formula from M_r; relate to NPK[6 marks]

Empirical formula of a two-nutrient fertiliser

A fertiliser compound contains, by mass, $21.2\%$ nitrogen, $6.8\%$ hydrogen, $23.5\%$ phosphorus and $48.5\%$ oxygen.

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

(b) The relative formula mass of the compound is $132$ . Deduce its molecular formula. [1]

(c) The compound is an ammonium salt. Write its formula showing the ammonium ions, and state which of the three NPK elements it does not supply. [2]

$A_r$ : H = 1; N = 14; O = 16; P = 31

Example 12ChallengeReacting masses in kg; limiting reactant; mass of excess; theoretical and percentage yield; suggest[7 marks]

Limiting reactant in making ammonium sulfate on a large scale

In a fertiliser plant, $17.0$ kg of ammonia reacts with $58.8$ kg of sulfuric acid: $2\text{NH}_3 + \text{H}_2\text{SO}_4 \rightarrow \text{(NH}_4\text{)}_2\text{SO}_4$

(a) Show that ammonia is the limiting reactant. [3]

(b) Calculate the mass of sulfuric acid left unreacted. [1]

(c) Calculate the maximum mass of ammonium sulfate that can form, in kg. [1]

(d) $59.4$ kg of ammonium sulfate is obtained. Calculate the percentage yield. [1]

(e) Suggest why the excess sulfuric acid must be removed before the fertiliser is sold. [1]

$A_r$ : H = 1; N = 14; O = 16; S = 32

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.

Chemistry of the Environment

States of Matter

Atoms, Elements and Compounds

Stoichiometry

Electrochemistry

Chemical Energetics

Chemical Energetics worked examples

Chemical Reactions

Acids, Bases and Salts

The Periodic Table

Metals

Organic Chemistry

Experimental Techniques and Chemical Analysis

Alternative to Practical

Alternative to Practical Skills worked examples

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Fertilisers Worked ExamplesIGCSE Chemistry 0620

Example 1CorePaper 2 MCQ: identify from formulae a mixture that supplies N, P and K[1 mark]

Which mixture supplies nitrogen, phosphorus and potassium? (multiple choice)

A gardener wants a fertiliser that provides all three of the elements nitrogen, phosphorus and potassium.

Which mixture should be used?

A $\text{NH}_4\text{NO}_3$ and KCl

B $\text{(NH}_4\text{)}_2\text{SO}_4$ and $\text{K}_3\text{PO}_4$

C $\text{KNO}_3$ and $\text{NH}_4\text{Cl}$

D $\text{(NH}_4\text{)}_3\text{PO}_4$ and NaCl

Example 2CoreState that ammonium salts and nitrates are fertilisers; which compound supplies N, P and K[4 marks]

Reading an NPK fertiliser label

The label shows the compounds in a bag of NPK fertiliser.

(a) Name the compound on the label that supplies

(i) phosphorus

(ii) potassium. [2]

(b) Ammonium salts and nitrates are both used as fertilisers. Name the compound on the label that is both an ammonium salt and a nitrate. [1]

(c) State why farmers add NPK fertilisers to soil. [1]

Example 3ExtendedPercentage composition by mass; mass of an element in a given mass of compound[5 marks]

Comparing the percentage of nitrogen in three fertilisers

Three compounds used in fertilisers are ammonium sulfate, $\text{(NH}_4\text{)}_2\text{SO}_4$ , ammonium phosphate, $\text{(NH}_4\text{)}_3\text{PO}_4$ , and potassium nitrate, $\text{KNO}_3$ .

(a) Calculate the percentage by mass of nitrogen in each compound. [3]

(b) Calculate the mass of nitrogen in a $50$ kg bag of pure ammonium sulfate. [1]

(c) A farmer needs a fertiliser that supplies nitrogen and potassium. Suggest which of the three compounds to use, and give a reason. [1]

$A_r$ : H = 1; N = 14; O = 16; P = 31; S = 32; K = 39

Example 4ExtendedPaper 6/4: read burette, titre, titration calculation with 2:1 ratio; obtain crystals of the salt[7 marks]

Making ammonium sulfate by titration: burette readings and concentration

The equation is $2\text{NH}_3 + \text{H}_2\text{SO}_4 \rightarrow \text{(NH}_4\text{)}_2\text{SO}_4$ .

(a) Record both burette readings and calculate the volume of acid added. [2]

(b) Calculate the concentration of the aqueous ammonia in $\text{mol/dm}^3$ . [3]

(c) Describe how the student could use this result to obtain pure, dry crystals of ammonium sulfate. [2]

1
(a) Read the bottom of the meniscus, to $0.05 \text{ cm}^3$ : start $= 1.20 \text{ cm}^3$ , end $= 22.80 \text{ cm}^3$ .
2
Volume of acid added = final − initial reading.
$22.80 - 1.20 = 21.60 \text{ cm}^3$
3
(b) Moles of sulfuric acid (convert $\text{cm}^3$ to $\text{dm}^3$ by $\div 1000$ ):
$n(\text{H}_2\text{SO}_4) = 0.100 \times \frac{21.60}{1000} = 2.16 \times 10^{-3} \text{ mol}$
4
Use the mole ratio $\text{NH}_3 : \text{H}_2\text{SO}_4 = 2 : 1$ .
$n(\text{NH}_3) = 2 \times 2.16 \times 10^{-3} = 4.32 \times 10^{-3} \text{ mol}$
5
Divide by the volume of ammonia in $\text{dm}^3$ .
$c = \frac{4.32 \times 10^{-3}}{0.0250} = 0.173 \text{ mol/dm}^3$
6
(c) Repeat the titration with $25.0 \text{ cm}^3$ of ammonia and $21.60 \text{ cm}^3$ of acid but without the indicator (which would contaminate the salt). Heat the solution to evaporate some water until the crystallisation point, cool to form crystals, then filter and dry them (between filter papers).

Answer

Examiner tip

Example 5ExtendedBases react with ammonium salts releasing ammonia; test for ammonia; equation and ionic equation[6 marks]

Why ammonium fertilisers should not be mixed with lime

(a) Name the gas that turns the litmus blue and the type of ion that fertiliser F must contain. [2]

(b) F is ammonium sulfate. Write the balanced equation for its reaction with calcium hydroxide. [2]

(c) Write the ionic equation for the reaction that produces the gas. [1]

(d) Explain why the farmer should not spread the two substances at the same time. [1]

NOT TO SCALE

Example 6ExtendedPaper 2 MCQ: qualitative analysis of a fertiliser (ammonium, sulfate, chloride tests)[1 mark]

Identifying a fertiliser from ion tests (multiple choice)

A solution of fertiliser G is tested.

Warmed with aqueous sodium hydroxide: a gas turns damp red litmus paper blue.
Acidified with dilute nitric acid, then aqueous barium nitrate added: white precipitate.
Acidified with dilute nitric acid, then aqueous silver nitrate added: no precipitate.

What is G?

A ammonium chloride

B ammonium sulfate

C potassium sulfate

D ammonium nitrate

Example 7ExtendedHaber process: sources, conditions, equation; neutralisation to make a nitrate fertiliser[8 marks]

From air and methane to ammonium nitrate: the Haber process link

The flow chart shows how fertiliser T is made.

(a) Name gas P, gas Q and raw material M. [3]

(b) State the typical temperature, pressure and catalyst used in the reactor. [2]

(c) Write the symbol equation, with state symbols, for the reaction in the reactor. [2]

(d) Product R reacts with acid S to form fertiliser T, which supplies nitrogen only. Name S and T. [1]

Example 8ExtendedGas volume (cm³) → moles → mass of product; percentage yield[5 marks]

Ammonia gas to ammonium nitrate: mass and percentage yield

Ammonia gas is passed into nitric acid: $\text{NH}_3\text{(g)} + \text{HNO}_3\text{(aq)} \rightarrow \text{NH}_4\text{NO}_3\text{(aq)}$

(a) $1800 \text{ cm}^3$ of ammonia, measured at r.t.p., reacts completely. Calculate the maximum mass of ammonium nitrate that can form. [3]

(b) The student obtains $5.10$ g of dry ammonium nitrate crystals. Calculate the percentage yield. [1]

(c) Suggest one reason why the yield is less than $100\%$ . [1]

$A_r$ : H = 1; N = 14; O = 16. The volume of one mole of any gas at r.t.p. is $24 \text{ dm}^3$ .

Example 9ExtendedPractical: gas syringe reading → moles of ammonia → mass of salt → percentage purity; sources of error[7 marks]

Purity of an ammonium sulfate fertiliser from a gas syringe

$\text{(NH}_4\text{)}_2\text{SO}_4\text{(s)} + 2\text{NaOH}\text{(aq)} \rightarrow \text{Na}_2\text{SO}_4\text{(aq)} + 2\text{NH}_3\text{(g)} + 2\text{H}_2\text{O}\text{(l)}$

(a) Read the volume of ammonia collected. [1]

(b) Calculate the mass of ammonium sulfate in the sample, and hence its percentage purity. The volume is measured at r.t.p. [4]

(c) Ammonia is very soluble in water. Explain how this affects the calculated purity, and suggest one improvement. [2]

$A_r$ : H = 1; N = 14; O = 16; S = 32

NOT TO SCALE

Example 10ExtendedRead a graph; percentage decrease; nitrates/phosphates cause deoxygenation; suggest recovery[6 marks]

Fertiliser run-off and dissolved oxygen in a river

A stream that drains farmland joins a river at distance $0$ km. The graph shows the dissolved oxygen in the river at different distances.

(a) State the dissolved oxygen concentration upstream of the stream, and the lowest value in the river. [2]

(b) Calculate the percentage decrease from the upstream value to the lowest value. [1]

(c) Name two substances from fertilisers that could cause this decrease, and state the harm caused. [2]

(d) Suggest why the dissolved oxygen increases again further downstream. [1]

Example 11ChallengeEmpirical formula from percentage composition; molecular formula from M_r; relate to NPK[6 marks]

Empirical formula of a two-nutrient fertiliser

A fertiliser compound contains, by mass, $21.2\%$ nitrogen, $6.8\%$ hydrogen, $23.5\%$ phosphorus and $48.5\%$ oxygen.

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

(b) The relative formula mass of the compound is $132$ . Deduce its molecular formula. [1]

(c) The compound is an ammonium salt. Write its formula showing the ammonium ions, and state which of the three NPK elements it does not supply. [2]

$A_r$ : H = 1; N = 14; O = 16; P = 31

Example 12ChallengeReacting masses in kg; limiting reactant; mass of excess; theoretical and percentage yield; suggest[7 marks]

Limiting reactant in making ammonium sulfate on a large scale

In a fertiliser plant, $17.0$ kg of ammonia reacts with $58.8$ kg of sulfuric acid: $2\text{NH}_3 + \text{H}_2\text{SO}_4 \rightarrow \text{(NH}_4\text{)}_2\text{SO}_4$

(a) Show that ammonia is the limiting reactant. [3]

(b) Calculate the mass of sulfuric acid left unreacted. [1]

(c) Calculate the maximum mass of ammonium sulfate that can form, in kg. [1]

(d) $59.4$ kg of ammonium sulfate is obtained. Calculate the percentage yield. [1]

(e) Suggest why the excess sulfuric acid must be removed before the fertiliser is sold. [1]

$A_r$ : H = 1; N = 14; O = 16; S = 32

Fertilisers Worked ExamplesIGCSE Chemistry 0620

Which mixture supplies nitrogen, phosphorus and potassium? (multiple choice)

Reading an NPK fertiliser label

Comparing the percentage of nitrogen in three fertilisers

Making ammonium sulfate by titration: burette readings and concentration

Why ammonium fertilisers should not be mixed with lime

Identifying a fertiliser from ion tests (multiple choice)

From air and methane to ammonium nitrate: the Haber process link

Ammonia gas to ammonium nitrate: mass and percentage yield

Purity of an ammonium sulfate fertiliser from a gas syringe

Fertiliser run-off and dissolved oxygen in a river

Empirical formula of a two-nutrient fertiliser

Limiting reactant in making ammonium sulfate on a large scale

More IGCSE Chemistry 0620 worked examples

Chemistry of the Environment

States of Matter

Atoms, Elements and Compounds

Stoichiometry

Electrochemistry

Chemical Energetics

Chemical Reactions

Acids, Bases and Salts

The Periodic Table

Metals

Organic Chemistry

Experimental Techniques and Chemical Analysis

Alternative to Practical

Fertilisers Worked ExamplesIGCSE Chemistry 0620

Which mixture supplies nitrogen, phosphorus and potassium? (multiple choice)

Reading an NPK fertiliser label

Comparing the percentage of nitrogen in three fertilisers

Making ammonium sulfate by titration: burette readings and concentration

Why ammonium fertilisers should not be mixed with lime

Identifying a fertiliser from ion tests (multiple choice)

From air and methane to ammonium nitrate: the Haber process link

Ammonia gas to ammonium nitrate: mass and percentage yield

Purity of an ammonium sulfate fertiliser from a gas syringe

Fertiliser run-off and dissolved oxygen in a river

Empirical formula of a two-nutrient fertiliser

Limiting reactant in making ammonium sulfate on a large scale

More IGCSE Chemistry 0620 worked examples

Chemistry of the Environment

States of Matter

Atoms, Elements and Compounds

Stoichiometry

Electrochemistry

Chemical Energetics

Chemical Reactions

Acids, Bases and Salts

The Periodic Table

Metals

Organic Chemistry

Experimental Techniques and Chemical Analysis

Alternative to Practical