Reacting mass calculations and limiting reagents
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Reacting mass calculations and limiting reagents — Free MYP5 Chemistry Practice Questions

1QuestionExcess reagent conceptConcept Practice
2 marks~3 minCriterion C
A student reacts magnesium ribbon (MgMg) with hydrochloric acid (HClHCl) in a beaker. The balanced equation is:

Mg+2HCl→MgCl2+H2Mg + 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]
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Solutions

2QuestionUsing balanced equations for mass calculationsAssessment Practice
3 marks~5 minCriterion A
A student investigates the reaction between calcium carbonate (CaCO3CaCO_3) and hydrochloric acid (HClHCl). She adds excess CaCO3CaCO_3 to 50 cm³ of 1.0 mol dm−3^{-3} HClHCl in a flask on a balance. Carbon dioxide (CO2CO_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.

CaCO3(s)+2HCl(aq)→CaCl2(aq)+H2O(l)+CO2(g)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 CO2CO_2 produced. [1]
c
Explain how the consumption of the limiting reagent causes the rate to decrease and eventually reach zero. [1]

Solutions

3QuestionUsing balanced equations for mass calculationsConcept Practice
3 marks~5 minCriterion B
Three combustion reactions are carried out in sealed (closed) containers.

Reaction A: CH4+2O2→CO2+2H2OCH_4 + 2O_2 \rightarrow CO_2 + 2H_2O
Reactants: 80.0 g — Products: 80.0 g

Reaction B: C2H6+3.5O2→2CO2+3H2OC_2H_6 + 3.5O_2 \rightarrow 2CO_2 + 3H_2O
Reactants: 142.0 g — Products: 142.0 g

Reaction C: C3H8+5O2→3CO2+4H2OC_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

4QuestionUsing balanced equations for mass calculationsAssessment Practice
8 marks~12 minCriterion D
A cement plant thermally decomposes calcium carbonate (CaCO3CaCO_3) from a limestone quarry according to:

CaCO3(s)→CaO(s)+CO2(g)CaCO_3(s) \rightarrow CaO(s) + CO_2(g)

The plant processes 1000 tonnes of CaCO3CaCO_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 CO2CO_2.

Molar masses: Ca=40Ca = 40 g/mol, C=12C = 12 g/mol, O=16O = 16 g/mol.
a
Calculate the mass of CO2CO_2, in tonnes, released per day. [2]
b
The plant introduces carbon capture technology, reducing CO2CO_2 emissions by 90%. Deduce the mass of CO2CO_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]
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Solutions

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