Balancing Equations and Conservation of Mass
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Balancing Equations and Conservation of Mass — Free MYP4 Chemistry Practice Questions

1QuestionMass Changes in Closed and Open SystemsConcept Practice
2 marks~3 minCriterion A
A student adds 5.0 g of sodium carbonate (Na2CO3Na_2CO_3) to a sealed flask containing excess hydrochloric acid (HClHCl). The reaction is:

Na2CO3(s)+2HCl(aq)→2NaCl(aq)+H2O(l)+CO2(g)Na_2CO_3(s) + 2HCl(aq) \rightarrow 2NaCl(aq) + H_2O(l) + CO_2(g)

The student records the total mass of the sealed flask before and after the reaction and finds it unchanged.
a
State the law that explains why the total mass remains unchanged. [1]
b
Explain why the mass would decrease if the flask were left open during the same reaction. [1]
Question diagram

Solutions

2QuestionBalancing Simple Chemical EquationsAssessment Practice
2 marks~3 minCriterion D
Natural gas power stations burn methane according to:

CH4+2O2→CO2+2H2OCH_4 + 2O_2 \rightarrow CO_2 + 2H_2O
a
Identify one environmental impact of methane combustion that can be deduced directly from this equation. [1]
b
Evaluate one limitation of using this equation to model the environmental impact of real-world natural gas combustion. [1]
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Solutions

3QuestionBalancing Complex Reactions with Polyatomic IonsAssessment Practice
4 marks~6 minCriterion C
Aluminium hydroxide reacts with nitric acid according to the unbalanced equation:

Al(OH)3(aq)+HNO3(aq)→Al(NO3)3(aq)+H2O(l)\text{Al(OH)}_3(aq) + \text{HNO}_3(aq) \rightarrow \text{Al(NO}_3)_3(aq) + \text{H}_2\text{O}(l)
a
Construct the balanced equation, treating the nitrate ion (NO3−\text{NO}_3^-) as a single unit. [2]
b
Justify why the coefficient of water in the balanced equation must be 3, using the conservation of mass. Your answer must account for both hydrogen and oxygen atoms. [2]
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Solutions

4QuestionBalancing Simple Chemical EquationsAssessment Practice
6 marks~9 minCriterion B
When a hydrocarbon burns completely in excess oxygen, the only products are carbon dioxide (CO2CO_2) and water (H2OH_2O). The table below shows combustion data for four alkanes.

HydrocarbonCH4CH_4C2H6C_2H_6C3H8C_3H_8C4H10C_4H_{10}
CO2CO_2 molecules produced1234
H2OH_2O molecules produced2345
a
Deduce the relationship between the molecular formula of a hydrocarbon and the number of CO2CO_2 and H2OH_2O molecules produced during complete combustion. [2]
b
Predict the number of CO2CO_2 and H2OH_2O molecules produced when one molecule of pentane (C5H12C_5H_{12}) burns completely in excess oxygen, and explain your reasoning. [2]
c
A student claims: "The pattern in the table is just a coincidence — it does not have to follow the law of conservation of mass." Justify whether this claim is correct or incorrect, using the balanced equation for the complete combustion of pentane as evidence. [2]

Solutions

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