Stoichiometry & Mole Concept — Free IB MYP5 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

2QuestionConverting between moles and number of particles (atoms, molecules, ions)Concept Practice

2 marks~3 minCriterion A

Hydrogen gas reacts with oxygen gas to produce water vapour in a combustion reaction studied in industrial fuel-cell technology.

a

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

b

Explain how Avogadro's constant connects the number of water molecules produced to the amount of water in moles. [1]

Solutions

3QuestionUsing 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

4QuestionMole ratios from balanced chemical equationsConcept Practice

2 marks~3 minCriterion A

The Haber process synthesises ammonia according to the following balanced equation:

N_2 + 3H_2 \rightarrow 2NH_3

Industrial plants use this reaction continuously, so precise mole ratios determine how much of each reactant is fed into the reactor.

a

State the mole ratio of

H_2

to

NH_3

in this reaction. [1]

b

A reactor is supplied with 12 mol of

H_2

. Deduce the maximum number of moles of

NH_3

that can be produced, assuming

N_2

is in excess. [1]

Solutions

5QuestionInterpreting coefficients in chemical equationsConcept Practice

2 marks~3 minCriterion C

The combustion of methane is represented by the equation:

CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O

The molecular models below show the four species involved, each labelled A–D. In these models, carbon atoms are black, oxygen atoms are red, and hydrogen atoms are white.

(a) Deduce which labelled molecule represents water (

H_2O

), and justify your answer by referring to both the molecular formula of water and the balanced equation. [2]

Solutions

6QuestionDilution concept and simple dilution calculationsConcept Practice

2 marks~3 minCriterion A

Three pieces of laboratory glassware are shown in the diagram: a beaker, a pipette, and a volumetric flask.

a

Identify the volumetric flask from the diagram. [1]

b

Explain why the volumetric flask, rather than the beaker, is used to prepare a diluted solution of precisely known concentration. [1]

Solutions

7QuestionUsing balanced equations for mass calculationsConcept Practice

3 marks~5 minCriterion B

Three combustion reactions are carried out in sealed (closed) containers.

Reaction A:

CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O

Reactants: 80.0 g — Products: 80.0 g

Reaction B:

C_2H_6 + 3.5O_2 \rightarrow 2CO_2 + 3H_2O

Reactants: 142.0 g — Products: 142.0 g

Reaction C:

C_3H_8 + 5O_2 \rightarrow 3CO_2 + 4H_2O

Reactants: 204.0 g — Products: 204.0 g

a

State the total mass of products in Reaction B. [1]

b

Identify the pattern shown by the mass data across all three reactions. [1]

c

Analyse why conducting these reactions in a closed system is essential for the pattern in (b) to be observed experimentally. [1]

Solutions

8QuestionExcess reagent conceptConcept Practice

2 marks~3 minCriterion C

A student reacts magnesium ribbon (

Mg

) with hydrochloric acid (

HCl

) in a beaker. The balanced equation is:

Mg + 2HCl \rightarrow MgCl_2 + H_2

After the reaction is complete, solid magnesium is still visible at the bottom of the beaker.

a

Identify the excess reagent in this reaction. [1]

b

Deduce which reagent was completely consumed, and explain how the observation described in the stem supports this conclusion. [1]

Solutions

9QuestionComparing reactions based on efficiency and waste productionConcept Practice

2 marks~3 minCriterion A

In the industrial Haber process, nitrogen and hydrogen react to produce ammonia. A reaction scheme shows a theoretical yield of 20.0 g of ammonia; the actual mass collected is 15.5 g.

(a) Calculate the percentage yield of this reaction. [2]

\text{Percentage yield} = \frac{\text{actual yield}}{\text{theoretical yield}} \times 100\%

Solutions

10QuestionRelationship between molar mass and one mole of substanceAssessment Practice

4 marks~6 minCriterion C

A laboratory balance shows two separate samples: one pile of 1 mole of carbon atoms (molar mass

12.01 \text{ g mol}^{-1}

) and one pile of 1 mole of magnesium atoms (molar mass

24.31 \text{ g mol}^{-1}

).

a

State the number of atoms present in each pile and identify the constant this number represents. [1]

b

Explain why the two piles contain equal numbers of atoms despite having different masses. [2]

c

A student claims: "Because magnesium atoms are heavier, 1 mole of magnesium must contain more atoms than 1 mole of carbon." Evaluate this claim. [1]

Solutions

11QuestionRelationship between molar mass and one mole of substanceAssessment Practice

8 marks~12 minCriterion D

The Haber-Bosch process produces ammonia (NH3) for fertilisers:

N_2 + 3H_2 \rightarrow 2NH_3

Molar masses: N2 = 28 g/mol, H2 = 2 g/mol, NH3 = 17 g/mol.

Global ammonia production relies on hydrogen derived from natural gas, generating approximately 1.9 tonnes of CO2 per tonne of NH3 produced.

a

Calculate the total mass of reactants needed to produce 34 g of NH3, and calculate the mass efficiency, defined as:

\text{mass efficiency} = \frac{\text{mass of product}}{\text{mass of reactants}} \times 100\%

[2]

b

Evaluate the societal benefits of the Haber-Bosch process against its environmental costs, using the CO2 emission data provided. [3]

c

Discuss the limitations of using molar mass and mass efficiency as the sole indicators of the sustainability of the Haber-Bosch process, identifying additional factors that should be considered. [3]

Solutions

12QuestionMr calculations for ionic vs covalent compoundsAssessment Practice

3 marks~5 minCriterion B

Four ionic compounds are listed below.

Compound	NaCl	MgO	CaCl $_2$	Al $_2$ O $_3$
$M_r$	58.5	40.3	111	102
Melting point (°C)	801	2852	772	2072

a

Deduce whether a linear relationship exists between

M_r

and melting point, using two specific examples from the data. [1]

b

Explain why melting point in ionic compounds depends on ionic charge and ionic radius rather than

M_r

. [1]

c

Analyse why MgO (melting point 2852°C) has a significantly higher melting point than CaCl

_2

(melting point 772°C), despite CaCl

_2

having a much larger

M_r

. [1]

Solutions

13QuestionMr calculations for ionic vs covalent compoundsAssessment Practice

12 marks~18 minCriterion A

The graph below shows the relationship between relative formula mass (

M_r

) and melting point for a series of ionic and covalent compounds.

a

Describe the trend shown in the graph for ionic compounds and for covalent compounds. [2]

b

The

M_r

of magnesium oxide (MgO) is 40 g/mol and the

M_r

of carbon dioxide (CO2) is 44 g/mol. Justify which compound has the higher melting point, referring to bonding and structure. [4]

c

Consider the two reactions below.

2\text{Mg}(s) + \text{O}_2(g) \rightarrow 2\text{MgO}(s) \quad \text{(Reaction 1)}

2\text{MgO}(s) \rightarrow 2\text{Mg}(s) + \text{O}_2(g) \quad \text{(Reaction 2)}

Evaluate which reaction is more likely to proceed at room temperature. In your answer, refer to the trend identified in part (a), the nature of the ionic lattice in MgO, and the concept of lattice energy. [6]

Solutions

14QuestionConcept of relative atomic mass (Ar) from periodic tableAssessment Practice

2 marks~3 minCriterion D

A mining company analyses a copper ore deposit in Chile. The local ore contains a higher proportion of

^{63}

Cu than the global average. The periodic table lists the relative atomic mass of copper as

A_r(\text{Cu}) = 63.55

.

a

State what the value

63.55

represents in terms of copper's two naturally occurring isotopes,

^{63}

Cu and

^{65}

Cu. [1]

b

Explain why using

A_r(\text{Cu}) = 63.55

to estimate the mass of copper extractable from this Chilean deposit may lead to an inaccuracy in the mining company's yield calculations. [1]

Solutions

15QuestionMr calculations for ionic vs covalent compoundsAssessment Practice

6 marks~9 minCriterion C

A student proposes the hypothesis: "The relative formula mass (

M_r

) calculated from atomic masses is equally accurate for ionic and covalent compounds." Mass spectrometry gives the following data:

NaCl — predicted $M_r$	$58.5$	experimental $M_r$ : $58.5$
$\text{CO}_2$ — predicted $M_r$	$44.0$	experimental $M_r$ : $44.0$
MgO — predicted $M_r$	$40.3$	experimental $M_r$ : $39.8$

a

Calculate the percentage discrepancy between the predicted and experimental

M_r

values for MgO. [1]

b

Explain why the experimental

M_r

values for NaCl and

\text{CO}_2

match their predicted values, while MgO shows a discrepancy. [3]

c

Evaluate whether the experimental evidence supports the hypothesis, and justify whether

M_r

calculations are equally reliable for ionic and covalent compounds. [2]

Solutions

16QuestionMole ratios from balanced chemical equationsAssessment Practice

8 marks~12 minCriterion D

The Haber-Bosch process synthesises ammonia (

\text{NH}_3

) for agricultural fertilizers:

\text{N}_2(g) + 3\text{H}_2(g) \rightarrow 2\text{NH}_3(g)

For every mole of

\text{NH}_3

produced, approximately 0.5 mol of methane (

\text{CH}_4

) is consumed to generate hydrogen, releasing 1.2 kg of

\text{CO}_2

. A farmer requires 340 kg of

\text{NH}_3

per hectare per year for optimal crop yields. Molar mass of

\text{NH}_3

= 17 g mol

^{-1}

.

a

Calculate the minimum moles of

\text{H}_2

required to produce 340 kg of

\text{NH}_3

. [2]

b

Deduce the total

\text{CO}_2

emissions (in kg) from producing this quantity of

\text{NH}_3

. [2]

c

Evaluate the societal and environmental trade-offs of using the Haber-Bosch process for fertilizer production, using your calculated data to support your conclusion. [4]

Solutions

17QuestionDilution concept and simple dilution calculationsAssessment Practice

3 marks~5 minCriterion B

A student dilutes 10 cm³ of a 1.0 mol dm⁻³ copper(II) sulfate solution by adding distilled water to reach four different final volumes.

Final volume (cm³)	20	40	60	80
Concentration (mol dm⁻³)	0.50	0.25	0.17	0.13

a

Calculate the number of moles of copper(II) sulfate present in the original 10 cm³ sample. [1]

b

Explain why the concentration decreases as the final volume increases. [1]

c

A student claims that doubling the final volume always halves the concentration. Evaluate this claim using the data provided. [1]

Solutions

18QuestionDefinition of concentration (mol/dm³ or g/dm³)Assessment Practice

5 marks~8 minCriterion C

A student prepares five copper(II) sulfate (

\text{CuSO}_4

) solutions by serial dilution of a

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

stock solution. Absorbance is measured at

635\ \text{nm}

.

Concentration ( $\text{mol dm}^{-3}$ )	$1.00$	$0.50$	$0.25$	$0.125$	$0.0625$
Absorbance	$1.20$	$0.60$	$0.55$	$0.30$	$0.15$

a

Calculate the expected absorbance for each diluted solution, assuming absorbance is directly proportional to concentration. [2]

b

Identify the anomalous data point and explain, using the definition of concentration in

\text{mol dm}^{-3}

, why the measured absorbance indicates the actual concentration differs from the intended value. [2]

c

Evaluate whether the anomaly is more likely caused by an error in volume measurement or mass measurement during the dilution, justifying your reasoning. [1]

Solutions

19QuestionCalculating moles from concentration and volumeAssessment Practice

4 marks~6 minCriterion D

A technician prepares a saline drip solution for a hospital ward. The solution contains sodium chloride (NaCl) dissolved in water. The volume of the solution is

0.50 \text{ L}

and its concentration is

2.0 \text{ mol/L}

.

a

Calculate the number of moles of NaCl in the solution using

n = c \times V

. [1]

b

A second bag contains

1.50 \text{ L}

of a NaCl solution with the same concentration. Deduce the total moles of NaCl across both bags. [1]

c

The recommended maximum daily intake of NaCl is

0.086 \text{ mol}

. Evaluate whether the combined NaCl from both bags poses a health concern, justifying your answer with a calculation. [2]

Solutions

20QuestionUsing balanced equations for mass calculationsAssessment Practice

3 marks~5 minCriterion A

A student investigates the reaction between calcium carbonate (

CaCO_3

) and hydrochloric acid (

HCl

). She adds excess

CaCO_3

to 50 cm³ of 1.0 mol dm

^{-3}

HCl

in a flask on a balance. Carbon dioxide (

CO_2

) escapes as it forms, causing a measurable mass loss. The graph shows mass loss rising steeply from 0 g, then levelling off at approximately 2.2 g after 150 seconds.

CaCO_3(s) + 2HCl(aq) \rightarrow CaCl_2(aq) + H_2O(l) + CO_2(g)

a

Describe the change in reaction rate shown by the graph. [1]

b

Identify the limiting reagent and explain why it controls the total mass of

CO_2

produced. [1]

c

Explain how the consumption of the limiting reagent causes the rate to decrease and eventually reach zero. [1]

Solutions

21QuestionUsing balanced equations for mass calculationsAssessment Practice

8 marks~12 minCriterion D

A cement plant thermally decomposes calcium carbonate (

CaCO_3

) from a limestone quarry according to:

CaCO_3(s) \rightarrow CaO(s) + CO_2(g)

The plant processes 1000 tonnes of

CaCO_3

per day (assumed 100% pure, complete reaction). The quarry provides 200 local jobs and improved infrastructure but destroys a 5-hectare forest and releases

CO_2

.

Molar masses:

Ca = 40

g/mol,

C = 12

g/mol,

O = 16

g/mol.

a

Calculate the mass of

CO_2

, in tonnes, released per day. [2]

b

The plant introduces carbon capture technology, reducing

CO_2

emissions by 90%. Deduce the mass of

CO_2

still released per day, and explain why the actual total emissions from the plant are likely higher than this value. [3]

c

Evaluate whether the economic and environmental trade-offs of this quarry and cement plant make it sustainable overall. In your answer, discuss the assumptions in the stoichiometric model and how they affect the reliability of the calculated emissions figure. [3]

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

23QuestionTheoretical yield vs actual yieldAssessment Practice

4 marks~6 minCriterion D

A pharmaceutical company synthesises paracetamol in two steps. Step 1 has a percentage yield of 72\%; Step 2 has a percentage yield of 88\%. The overall percentage yield equals the product of the two individual yields.

a

Calculate the overall percentage yield of the synthesis. [1]

b

A factory running this process handles 500 kg of starting material per batch. Calculate the mass of paracetamol produced per batch and deduce the mass of material that does not become product. [2]

c

Discuss the environmental and health implications of the unreacted material identified in (b), considering waste treatment requirements and risks to nearby communities. [1]

Solutions

24QuestionDefinition 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 the 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 at 3.5 g.

a

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

\text{CO}_2

. Show all working. [1]

b

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

c

A second trial is run at a higher temperature. The curve reaches the same plateau value of 3.5 g but in less time. Evaluate whether this result supports the conclusion that increasing temperature increases percentage yield. [1]

Solutions

Stoichiometry & Mole Concept — Free MYP5 Chemistry Practice Questions