Stoichiometry & Mole Concept — Free IB MYP4 Chemistry Questions

1QuestionDefinition of a mole and amount of substanceConcept Practice

3 marks~5 minCriterion B

The table below shows data for one mole of four substances.

Substance — Mass of one mole (g) — Number of particles

Carbon (C) — 12.0 —

6.02 \times 10^{23}

Water (H

_2

O) — 18.0 —

6.02 \times 10^{23}

Sodium chloride (NaCl) — 58.5 —

6.02 \times 10^{23}

Oxygen (O

_2

) — 32.0 —

6.02 \times 10^{23}

a

State what the data shows about the number of particles present in one mole of each substance. [1]

b

Identify the pattern in molar mass shown by the four substances. [1]

c

Explain what the relationship between molar mass and number of particles reveals about the definition of a mole. [1]

Solutions

2QuestionCounting particles in compounds vs elementsConcept Practice

2 marks~3 minCriterion A

Iron rusts when exposed to air, forming iron(III) oxide — a process relevant to infrastructure maintenance worldwide.

a

Construct the balanced chemical equation for iron reacting with oxygen to form iron(III) oxide, including state symbols. [1]

b

Identify the type of particle (atom, molecule, or formula unit) that represents each reactant and the product in this reaction, and explain why iron(III) oxide is described using formula units rather than molecules. [1]

Solutions

3QuestionDifference between molecular mass and formula massConcept Practice

2 marks~3 minCriterion A

The diagram shows a sodium chloride (

\text{NaCl}

) crystal lattice and a water (

\text{H}_2\text{O}

) molecule.

a

State which substance has a molecular mass and which has a formula mass. [1]

b

Explain, with reference to bonding and structure, why different mass terminology is used for each substance. [1]

Solutions

4QuestionUsing formula: moles = mass ÷ molar massConcept Practice

2 marks~3 minCriterion B

The table below shows the first five straight-chain alkanes and their molar masses.

Alkane	Methane	Ethane	Propane	Butane	Pentane
Molecular formula	$CH_4$	$C_2H_6$	$C_3H_8$	$C_4H_{10}$	$C_5H_{12}$
Molar mass (g/mol)	16	30	44	58	72

a

Deduce the molar mass of hexane,

C_6H_{14}

, without a periodic table, showing your reasoning. [1]

b

Explain why each successive alkane in this series has a molar mass exactly 14 g/mol greater than the previous one. [1]

Solutions

5QuestionConverting between mass and molesConcept Practice

3 marks~5 minCriterion A

The graph below shows the mass of magnesium ribbon (g) against time (s) as it reacts with excess hydrochloric acid. The magnesium starts at

1.00\ \text{g}

and its mass approaches

0.00\ \text{g}

after approximately

120\ \text{s}

.

a

Describe the trend shown in the graph. [1]

b

Explain why the rate of reaction decreases over time. [1]

c

Construct the balanced chemical equation for this reaction, including state symbols. [1]

Solutions

6QuestionCalculating moles from concentration and volumeConcept Practice

3 marks~5 minCriterion A

When 50 cm³ of 1.0 mol/dm³ hydrochloric acid reacts with excess calcium carbonate chips, the mass of CO₂ produced increases rapidly at first, then more slowly after 2 minutes.

a

Calculate the initial number of moles of HCl present. [1]

b

Explain how the number of moles of HCl changes during the reaction and what this means for the concentration of HCl in solution. [1]

c

Using collision theory, explain why the rate of reaction decreases as the reaction proceeds. [1]

Solutions

7QuestionUsing balanced equations for mass calculationsConcept Practice

4 marks~6 minCriterion A

Natural gas appliances release carbon dioxide into the atmosphere. In one such appliance, propane undergoes complete combustion:

C_3H_8 + 5O_2 \rightarrow 3CO_2 + 4H_2O

Atomic masses: C = 12.0 g/mol, H = 1.0 g/mol, O = 16.0 g/mol.

A gas canister contains 44.0 g of propane (

C_3H_8

), which is burned completely in excess oxygen.

a

Calculate the molar mass of propane (

C_3H_8

) and hence the number of moles present. [1]

b

Deduce the mass of carbon dioxide (

CO_2

) produced. [2]

c

Evaluate whether doubling the mass of propane burned would double the mass of

CO_2

produced, assuming oxygen remains in excess. Justify your answer using the stoichiometry of the reaction. [1]

Solutions

8QuestionConcept of atom economy in reactionsConcept Practice

2 marks~3 minCriterion A

Ethanol (

C_2H_5OH

) is produced industrially by reacting ethene (

C_2H_4

) with steam (

H_2O

) in a single-step addition reaction:

C_2H_4 + H_2O \rightarrow C_2H_5OH

Molar masses:

C_2H_4 = 28\ \text{g/mol}

,

H_2O = 18\ \text{g/mol}

,

C_2H_5OH = 46\ \text{g/mol}

Atom economy

= \dfrac{\text{mass of desired product}}{\text{total mass of reactants}} \times 100\%

(a) Calculate the atom economy for this reaction. [2]

Solutions

9QuestionConverting between moles and number of particles (atoms, molecules, ions)Assessment Practice

2 marks~3 minCriterion C

Ibuprofen (C

_{13}

H

_{18}

O

_2

) is manufactured in batches. A quality-control chemist uses Avogadro's constant (

N_A = 6.02 \times 10^{23}\ \text{mol}^{-1}

) to estimate the number of ibuprofen molecules in a 0.500 mol sample. Carbon naturally exists as the isotopes carbon-12 and carbon-13.

Identify one limitation of using Avogadro's constant to determine the exact number of ibuprofen molecules in this sample, and explain how the presence of carbon isotopes causes this limitation. [2]

Solutions

10QuestionDefinition of a mole and amount of substanceAssessment Practice

6 marks~9 minCriterion D

A fertilizer company proposes building an ammonia plant using the Haber process:

N_2(g) + 3H_2(g) \rightleftharpoons 2NH_3(g)

The plant would produce 500,000 tonnes of NH3 annually. Hydrogen is sourced from natural gas, releasing 1.2 million tonnes of CO2 per year. The ammonia would be converted into fertilizers capable of sustaining an additional 2 million people.

Molar mass of NH3 = 17.0 g mol

^{-1}

; molar mass of CO2 = 44.0 g mol

^{-1}

.

a

Calculate the number of moles of NH3 produced and CO2 emitted annually. [2]

b

Explain how the mole concept quantifies the industrial scale of both production and emissions. [2]

c

Evaluate the societal and environmental trade-offs of this proposal, identifying two limitations of using stoichiometric mole ratios to model real-world Haber process efficiency. [2]

Solutions

11QuestionCalculation of relative formula mass (Mr) for compoundsAssessment Practice

3 marks~5 minCriterion B

The table below shows the molecular formulae and relative formula masses (

M_r

) of four alkanes.

Alkane	Methane	Ethane	Propane	Butane
Molecular formula	$CH_4$	$C_2H_6$	$C_3H_8$	$C_4H_{10}$
$M_r$	16	30	44	58

a

State the trend in

M_r

as the number of carbon atoms increases. [1]

b

Each successive alkane differs by one

CH_2

unit. Calculate the

M_r

of this

CH_2

unit and show how it accounts for the increase in

M_r

at each step. [1]

c

Pentane (

C_5H_{12}

) is the next alkane in the series. Using the pattern identified above, deduce its

M_r

and explain whether this value is consistent with the homologous series trend. [1]

Solutions

12QuestionCalculation of relative formula mass (Mr) for compoundsAssessment Practice

3 marks~5 minCriterion C

A graph of mass (g) against number of moles for a pure compound produces a straight line through the origin with a gradient of 44.

a

Using the equation

\text{mass} = \text{moles} \times M_r

, explain what the gradient of this graph represents. [1]

b

Calculate the

M_r

of the compound. [1]

c

Three compounds share this

M_r

:

CO_2

,

N_2O

, and

C_3H_8

. Evaluate whether the graph alone is sufficient to identify which compound is present. Justify your answer. [1]

Solutions

13QuestionConcept of relative atomic mass (Ar) from periodic tableAssessment Practice

6 marks~9 minCriterion D

Natural uranium consists of two isotopes: uranium-235 (mass 235.04) and uranium-238 (mass 238.05). The periodic table lists uranium's relative atomic mass (

A_r

) as 238.03. This value is derived from uranium ore samples collected worldwide.

a

Let

x

represent the fractional abundance of uranium-235. Show that the percentage abundance of uranium-235 in natural uranium is approximately 0.66%. [2]

b

Explain why a uranium ore sample from a single mine may have a slightly different

A_r

value than the one listed in the periodic table. [2]

c

Evaluate one limitation of using the periodic table's

A_r

value when assessing the radioactivity of a specific uranium ore sample, and identify one environmental risk associated with uranium mining. [2]

Solutions

14QuestionInterpreting coefficients in chemical equationsAssessment Practice

6 marks~9 minCriterion D

A factory burns methane (CH

_4

) in excess oxygen each day:

\text{CH}_4 + 2\text{O}_2 \rightarrow \text{CO}_2 + 2\text{H}_2\text{O}

The factory consumes 160 kg of CH

_4

daily. The local authority requires the factory to report its daily CO

_2

output and assess its environmental significance.

a

Show that the mass of CO

_2

produced daily is 440 kg. [2]

b

Explain how the daily CO

_2

emissions from this factory contribute to the greenhouse effect. [2]

c

Evaluate the reliability of using this balanced equation alone to predict the factory's actual daily CO

_2

emissions. [2]

Solutions

15QuestionConverting between mass and molesAssessment Practice

4 marks~6 minCriterion C

When magnesium burns in oxygen, the reaction follows the equation

2\text{Mg} + \text{O}_2 \rightarrow 2\text{MgO}.

Molar masses:

\text{Mg} = 24.3 \ \text{g mol}^{-1}

;

\text{O} = 16.0 \ \text{g mol}^{-1}

;

\text{MgO} = 40.3 \ \text{g mol}^{-1}

.

a

Calculate the mass of magnesium required to produce

60.0 \ \text{g}

of MgO. [2]

b

A student claims: "Because Mg and MgO are in a 1 : 1 molar ratio, the mass of Mg needed always equals the mass of MgO produced." Using your calculated values, explain why this claim is incorrect and identify the chemical reason for the mass difference. [2]

Solutions

16QuestionCalculating moles from concentration and volumeAssessment Practice

2 marks~3 minCriterion B

A student prepares four solutions by diluting a

2.0 \ \text{mol dm}^{-3}

NaCl stock solution to a total volume of

100 \ \text{cm}^3

each time.

Volume of stock solution ( $\text{cm}^3$ )	10	20	30	40
Final concentration ( $\text{mol dm}^{-3}$ )	0.20	0.40	0.60	0.80

a

Deduce the final concentration when

50 \ \text{cm}^3

of stock solution is used. [1]

b

Explain why this relationship between volume of stock solution and final concentration only holds when the total final volume remains constant. [1]

Solutions

17QuestionCalculating moles from concentration and volumeAssessment Practice

6 marks~9 minCriterion C

A student titrates

25.0 \ \text{cm}^3

of hydrochloric acid (

\text{HCl}

) against

0.100 \ \text{mol dm}^{-3}

sodium hydroxide (

\text{NaOH}

) solution, using phenolphthalein as the indicator. Five runs are recorded below.

Run	1	2	3	4	5
Initial reading (cm³)	0.00	0.10	0.20	0.00	0.15
Final reading (cm³)	26.50	26.60	27.80	26.40	26.55
Titre volume (cm³)	26.50	26.50	27.60	26.40	26.40

a

Identify the anomalous titre value and justify your choice. [1]

b

Calculate the mean titre volume in

\text{cm}^3

, excluding the anomalous value. Show your working. [2]

c

Using your mean titre volume, calculate the number of moles of

\text{NaOH}

that reacted. [2]

d

Deduce the number of moles of

\text{HCl}

present in the

25.0 \ \text{cm}^3

sample, given that the neutralisation reaction is:

\text{NaOH} + \text{HCl} \rightarrow \text{NaCl} + \text{H}_2\text{O}

Justify your reasoning using the mole ratio. [1]

Solutions

18QuestionDilution concept and simple dilution calculationsAssessment Practice

8 marks~12 minCriterion D

A chemical factory discharges sulfuric acid waste at a concentration of

0.050 \ \text{mol dm}^{-3}

and a flow rate of

2.0 \ \text{m}^3 \text{h}^{-1}

into a river. The river's upstream flow rate is

500 \ \text{m}^3 \text{h}^{-1}

and initially contains no acid. Assume perfect mixing and no chemical reactions unless stated otherwise.

a

Calculate the concentration of sulfuric acid in the river downstream of the discharge point. [2]

b

Deduce whether the downstream concentration would be higher or lower than your calculated value during a period of heavy rainfall. Justify your answer with reference to the dilution formula. [2]

c

Evaluate the suitability of the model

C_1V_1 = C_2V_2

for predicting actual acid concentrations in this river, discussing the assumption of no chemical reactions and one further limitation. [4]

Solutions

19QuestionStep-by-step stoichiometric calculations (mass → moles → ratio → mass)Assessment Practice

3 marks~5 minCriterion B

Four experiments record the masses of magnesium and oxygen that react completely to form magnesium oxide (MgO).

Experiment	1	2	3	4
Mass of Mg (g)	3.0	6.0	9.0	12.0
Mass of O $_2$ (g)	2.0	4.0	6.0	8.0
Mass of MgO (g)	5.0	10.0	15.0	20.0

a

Calculate the mass ratio of Mg to O

_2

for each experiment. [1]

b

Describe the pattern shown by the four mass ratios. [1]

c

Explain why the mass ratio of Mg to O

_2

remains constant regardless of the quantity of MgO formed. [1]

Solutions

20QuestionReal-life applications (industrial reactions, efficiency)Assessment Practice

2 marks~3 minCriterion D

The Haber process synthesises ammonia (

NH_3

) for use in fertilizers:

N_2 + 3H_2 \rightarrow 2NH_3

In an industrial reactor, 28 g of

N_2

and 10 g of

H_2

are combined. Stoichiometric calculations predict a theoretical yield of

NH_3

based on the limiting reagent.

Explain one real-world limitation of using the stoichiometric model to predict the actual yield of

NH_3

in this industrial process, and describe how it affects the yield. [2]

Solutions

21QuestionExcess reagent conceptAssessment Practice

3 marks~5 minCriterion C

In the Haber process, nitrogen gas reacts with hydrogen gas to produce ammonia:

N_2(g) + 3H_2(g) \rightarrow 2NH_3(g)

A student fixes the amount of

N_2

at 0.50 mol and varies the amount of

H_2

added. The mass of

NH_3

produced is recorded below.

$H_2$ added (mol)	0	0.50	1.0	1.5	2.0
Mass of $NH_3$ produced (g)	0	5.7	11.3	17.0	17.0

a

Calculate the moles of

NH_3

produced when 1.5 mol of

H_2

is added. [1]

b

Explain why no additional

NH_3

is produced when the amount of

H_2

added exceeds 1.5 mol. [1]

c

A second student suggests that adding more

H_2

beyond 1.5 mol could increase the yield of

NH_3

if the temperature is lowered simultaneously. Evaluate this claim using your understanding of limiting reagents and stoichiometry. [1]

Solutions

22QuestionComparing reactions based on efficiency and waste productionAssessment Practice

4 marks~6 minCriterion B

Four industrial synthesis routes to ibuprofen are summarised below.

Route	A	B	C	D
Atom economy (%)	77.3	60.1	40.5	99.1
Waste per kg of ibuprofen (kg)	0.293	0.664	1.47	0.009

a

Describe the pattern between atom economy and waste mass per kg of ibuprofen produced. Support your answer with data from two routes. [1]

b

Deduce a general rule linking atom economy to waste production, and explain the chemical reason for this relationship. [2]

c

Using Route C data, justify your rule by calculating the expected waste mass per kg of ibuprofen and comparing it to the value in the table. [1]

Solutions

23QuestionDefinition and calculation of percentage yieldAssessment Practice

3 marks~5 minCriterion C

During an industrial trial, calcium carbonate is thermally decomposed:

\text{CaCO}_3\text{(s)} \rightarrow \text{CaO(s)} + \text{CO}_2\text{(g)}

The graph shows mass of

\text{CO}_2\text{(g)}

produced over time. A horizontal dashed line marks the theoretical yield at 4.4 g; the curve rises then plateaus.

a

State the formula for percentage yield and calculate the percentage yield of

\text{CO}_2

, given that the actual yield read from the plateau is 3.5 g. Show all working. [2]

b

The theoretical yield assumes complete decomposition. Deduce one reason why the actual yield is lower than the theoretical yield in this reaction, and explain how this reason is consistent with the shape of the graph. [1]

Solutions

24QuestionDefinition and calculation of percentage yieldAssessment Practice

6 marks~9 minCriterion D

A pharmaceutical company synthesises amoxicillin with a percentage yield of 62%. The reaction used to produce amoxicillin has an atom economy of 38%. For every kilogram of amoxicillin produced, 8 kg of organic solvent waste is generated.

a

Show that the mass of organic solvent waste produced when the company targets 50 kg of amoxicillin is 400 kg. [1]

b

Using the percentage yield and the atom economy values, explain how each metric contributes to the total waste generated in this process. [2]

c

Evaluate whether percentage yield or atom economy is the more useful metric for a chemist designing a new, greener synthesis route for amoxicillin. [3]

Solutions

Stoichiometry & Mole Concept — Free MYP4 Chemistry Practice Questions