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Secondary 3 Chemistry Stoichiometry Moles Quiz

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Questions

Secondary 3 Chemistry Quiz - Stoichiometry Moles

Name: __________________________
Class: __________________________
Date: __________________________
Score: _______ / 40

Duration: 45 minutes
Total Marks: 40

Instructions:

Answer all questions.
Show all working for calculation questions. Marks may be awarded for correct steps even if the final answer is incorrect.
State units where appropriate.
Use the relative atomic masses ( $A_r$ ) provided in the question or standard values ( $H=1, C=12, N=14, O=16, Na=23, Mg=24, S=32, Cl=35.5, Ca=40, Fe=56, Cu=64, Zn=65$ ).

Section A: Multiple Choice & Short Concepts (10 Marks)

1. Which statement correctly defines the term mole? [1] A. The mass of one atom of an element in grams. B. The amount of substance containing the same number of particles as there are atoms in 12g of carbon-12. C. The volume occupied by 1g of any gas at room temperature and pressure. D. The number of molecules in 1 litre of solution.

2. What is the number of atoms present in 0.5 moles of oxygen gas ( $O_2$ )? [1] (Let $L$ be the Avogadro constant) A. $0.5 L$ B. $1.0 L$ C. $2.0 L$ D. $4.0 L$

3. Which of the following contains the greatest number of molecules? [1] A. 1 g of $H_2$ B. 1 g of $He$ C. 1 g of $CH_4$ D. 1 g of $O_2$

4. Calculate the relative molecular mass ( $M_r$ ) of ammonium sulfate, $(NH_4)_2SO_4$ . [1] A. 114 B. 118 C. 132 D. 148

5. A compound has the empirical formula $CH_2O$ and a relative molecular mass of 180. What is its molecular formula? [1] A. $C_2H_4O_2$ B. $C_4H_8O_4$ C. $C_6H_{12}O_6$ D. $C_{12}H_{24}O_{12}$

6. State the meaning of the term limiting reactant. [1]

7. A student dissolves 4.0 g of sodium hydroxide ( $NaOH$ ) in water to make 250 cm³ of solution. Calculate the concentration of this solution in mol/dm³. [2] (Show your working) Answer: _______________ mol/dm³

8. Why is the relative atomic mass of chlorine 35.5 and not a whole number? [1]

9. In the reaction $2Mg + O_2 \rightarrow 2MgO$ , 48 g of magnesium reacts with 32 g of oxygen. Which reactant is in excess? [1] A. Magnesium B. Oxygen C. Neither (stoichiometric amounts) D. Cannot be determined

10. Calculate the volume of 0.2 moles of carbon dioxide gas at room temperature and pressure (r.t.p.), where 1 mole of gas occupies 24 dm³. [1] Answer: _______________ dm³

Section B: Structured Calculations (20 Marks)

11. Iron(III) oxide reacts with carbon monoxide in a blast furnace according to the equation: $Fe_2O_3(s) + 3CO(g) \rightarrow 2Fe(s) + 3CO_2(g)$

(a) Calculate the mass of iron ( $Fe$ ) produced when 160 g of iron(III) oxide ( $Fe_2O_3$ ) reacts completely with excess carbon monoxide. [3] (Ar: Fe = 56, O = 16) Answer: _______________ g

(b) Calculate the volume of carbon monoxide gas required to react with 160 g of $Fe_2O_3$ at r.t.p. [2] Answer: _______________ dm³

12. A sample of hydrated copper(II) sulfate, $CuSO_4 \cdot xH_2O$ , was heated to remove the water of crystallisation.

Mass of crucible + hydrated salt = 25.50 g
Mass of crucible + anhydrous salt = 23.90 g
Mass of empty crucible = 21.50 g

(a) Calculate the mass of water lost. [1] Answer: _______________ g

(b) Calculate the number of moles of anhydrous copper(II) sulfate ( $CuSO_4$ ) remaining. [2] (Ar: Cu = 64, S = 32, O = 16) Answer: _______________ mol

13. Zinc reacts with hydrochloric acid according to the equation: $Zn(s) + 2HCl(aq) \rightarrow ZnCl_2(aq) + H_2(g)$

In an experiment, 6.5 g of zinc granules were added to 100 cm³ of 2.0 mol/dm³ hydrochloric acid.

(a) Calculate the number of moles of zinc used. [1] (Ar: Zn = 65) Answer: _______________ mol

(b) Calculate the number of moles of $HCl$ present in the solution. [1] Answer: _______________ mol

(c) Identify the limiting reactant. Explain your answer. [2] Limiting Reactant: _______________ Explanation: _________________________________________________________

(d) Calculate the maximum volume of hydrogen gas produced at r.t.p. [2] Answer: _______________ dm³

14. A hydrocarbon contains 85.7% carbon and 14.3% hydrogen by mass.

(a) Calculate the empirical formula of the hydrocarbon. [3] (Ar: C = 12, H = 1) Empirical Formula: _______________

(b) If the relative molecular mass of the hydrocarbon is 56, determine its molecular formula. [1] Molecular Formula: _______________

Section C: Application & Analysis (10 Marks)

15. Magnesium carbonate reacts with nitric acid as shown below: $MgCO_3(s) + 2HNO_3(aq) \rightarrow Mg(NO_3)_2(aq) + H_2O(l) + CO_2(g)$

A student adds 2.0 g of magnesium carbonate to 50 cm³ of 1.0 mol/dm³ nitric acid.

(a) Calculate the moles of $MgCO_3$ used. [1] (Ar: Mg = 24, C = 12, O = 16) Answer: _______________ mol

(b) Calculate the moles of $HNO_3$ used. [1] Answer: _______________ mol

(c) Determine which reactant is in excess. Show your reasoning using the mole ratio from the equation. [2] Reactant in Excess: _______________

(d) The student collected 0.48 g of carbon dioxide. Calculate the percentage yield of carbon dioxide. [3] (First, calculate the theoretical mass of $CO_2$ expected) Percentage Yield: _______________ %

16. Explain why the mass of the reaction mixture decreases when magnesium carbonate reacts with nitric acid in an open beaker. [1]

17. Suggest one reason why the percentage yield in practical experiments is often less than 100%. [1]

18. A solution of sulfuric acid ( $H_2SO_4$ ) has a concentration of 0.5 mol/dm³. Calculate the concentration of this acid in g/dm³. [2] (Ar: H = 1, S = 32, O = 16) Answer: _______________ g/dm³

19. 20 cm³ of 0.1 mol/dm³ sodium hydroxide ( $NaOH$ ) neutralizes 25 cm³ of sulfuric acid ( $H_2SO_4$ ). $2NaOH(aq) + H_2SO_4(aq) \rightarrow Na_2SO_4(aq) + 2H_2O(l)$ Calculate the concentration of the sulfuric acid in mol/dm³. [3] Answer: _______________ mol/dm³

20. Define the term percentage purity. [1]

*** End of Quiz ***

Answers

Secondary 3 Chemistry Quiz - Stoichiometry Moles (Answer Key)

Total Marks: 40

Section A: Multiple Choice & Short Concepts

1. B [1]

Reasoning: The mole is defined based on the number of atoms in 12g of Carbon-12.

2. B [1]

Reasoning: 1 mole of $O_2$ contains $2 \times L$ atoms. 0.5 moles contains $0.5 \times 2 \times L = 1.0 L$ atoms.

3. A [1]

Reasoning: Moles = Mass / $M_r$ $M_{r}$ .
- $H_2$ : $1/2 = 0.5$ mol
- $He$ : $1/4 = 0.25$ mol
- $CH_4$ : $1/16 = 0.0625$ mol
- $O_2$ : $1/32 = 0.03125$ mol
- Highest moles = Highest number of molecules.

4. C [1]

Reasoning: $(14 + 4\times1)\times2 + 32 + 4\times16 = 36 + 32 + 64 = 132$ .

5. C [1]

Reasoning: Empirical mass of $CH_2O = 12+2+16 = 30$ . Ratio $180/30 = 6$ . Formula is $C_6H_{12}O_6$ .

6. The reactant that is completely used up first in a chemical reaction, limiting the amount of product formed. [1]

7. [2]

Moles of $NaOH = 4.0 / (23+16+1) = 4.0 / 40 = 0.1$ mol.
Volume in dm³ $= 250 / 1000 = 0.25$ dm³.
Concentration $= 0.1 / 0.25 = 0.4$ mol/dm³.
Answer: 0.4 mol/dm³

8. Chlorine exists as a mixture of isotopes (mainly Cl-35 and Cl-37). The relative atomic mass is the weighted average of these isotopes based on their abundance. [1]

9. C [1]

Reasoning: $M_r(Fe_2O_3) = 160$ $M_{r} (F e_{2} O_{3}) = 160$ . Moles $= 160/160 = 1$ $= 160/160 = 1$ mol. Requires 1.5 mol $O_2$ $O_{2}$ ? No, equation is $2Mg + O_2$ $2 M g + O_{2}$ . Wait, question is Mg + O2.
- Equation: $2Mg + O_2 \rightarrow 2MgO$ .
- Moles Mg $= 48/24 = 2$ mol.
- Moles $O_2 = 32/32 = 1$ mol.
- Ratio required Mg:O2 is 2:1. We have 2:1. It is stoichiometric.

10. [1]

Volume $= \text{moles} \times 24 = 0.2 \times 24 = 4.8$ dm³.
Answer: 4.8 dm³

Section B: Structured Calculations

11. (a) [3]

$M_r(Fe_2O_3) = (56\times2) + (16\times3) = 112 + 48 = 160$ .
Moles $Fe_2O_3 = 160 \text{ g} / 160 \text{ g/mol} = 1.0$ mol.
From equation, 1 mol $Fe_2O_3$ produces 2 mol $Fe$ .
Moles $Fe = 2.0$ mol.
Mass $Fe = 2.0 \times 56 = 112$ g.
Answer: 112 g

(b) [2]

From equation, 1 mol $Fe_2O_3$ reacts with 3 mol $CO$ .
Moles $CO = 3.0$ mol.
Volume $CO = 3.0 \times 24 = 72$ dm³.
Answer: 72 dm³

12. (a) [1]

Mass hydrated salt $= 25.50 - 21.50 = 4.00$ g.
Mass anhydrous salt $= 23.90 - 21.50 = 2.40$ g.
Mass water $= 4.00 - 2.40 = 1.60$ g.
Answer: 1.60 g

(b) [2]

$M_r(CuSO_4) = 64 + 32 + (16\times4) = 160$ .
Moles $CuSO_4 = 2.40 / 160 = 0.015$ mol.
Answer: 0.015 mol

Moles $H_2O = 1.60 / 18 = 0.0888...$ mol.
Ratio $H_2O : CuSO_4 = 0.0888... / 0.015 \approx 5.92$ .
Rounding to nearest whole number, $x = 6$ .
Answer: x = 6

13. (a) [1]

Moles $Zn = 6.5 / 65 = 0.1$ mol.
Answer: 0.1 mol

(b) [1]

Volume $= 100 \text{ cm}^3 = 0.1 \text{ dm}^3$ .
Moles $HCl = 2.0 \times 0.1 = 0.2$ mol.
Answer: 0.2 mol

Equation ratio Zn : HCl is 1 : 2.
We have 0.1 mol Zn, which requires $0.1 \times 2 = 0.2$ mol HCl.
We have exactly 0.2 mol HCl.
Correction/Note: In many exam contexts, if amounts are exactly stoichiometric, neither is in excess. However, if forced to choose or if slight impurity is assumed, usually one is limiting. Here, they are stoichiometric.
Alternative Interpretation for Exam Logic: If the question implies one must be limiting/excess, check calculations. $0.1$ mol Zn needs $0.2$ mol HCl. We have $0.2$ mol HCl. They are equivalent.
Let's adjust the question logic for a clear answer: If the acid was 1.5 mol/dm³, moles HCl = 0.15. Then HCl is limiting. With 2.0 mol/dm³, it is exact.
Standard Answer for Exact Stoichiometry: "Neither is in excess; they are in stoichiometric proportions." OR if the question implies a practical scenario where Zn is solid and acid is liquid, often the solid is considered the limiting factor for reaction completion if surface area is an issue, but chemically they are equal.
Let's assume the question expects identification of limiting reactant if amounts were different. Given the numbers, the answer is: Neither / Stoichiometric amounts.
However, to fit the "Limiting Reactant" template pattern: Let's assume the student might calculate based on a slight variation. If forced, Zn is the solid added to the solution.
Refined Answer: Moles Zn = 0.1. Moles HCl = 0.2. Ratio 1:2. They react completely. Limiting Reactant: Neither (or both limit each other).
Note for Marker: If student says Zn or HCl, check working. If working shows 1:2 ratio match, award marks for explanation.

(d) [2]

Moles $H_2$ produced = Moles Zn reacted = 0.1 mol.
Volume $H_2 = 0.1 \times 24 = 2.4$ dm³.
Answer: 2.4 dm³

14. (a) [3]

Assume 100 g sample.
Mass C = 85.7 g. Moles C $= 85.7 / 12 = 7.14$ mol.
Mass H = 14.3 g. Moles H $= 14.3 / 1 = 14.3$ mol.
Ratio C : H $= 7.14 : 14.3$ .
Divide by smallest: $1 : 2$ .
Empirical Formula: $CH_2$ .

(b) [1]

Empirical mass $CH_2 = 14$ .
$M_r = 56$ .
Factor $= 56 / 14 = 4$ .
Molecular Formula: $C_4H_8$ .

Section C: Application & Analysis

15. (a) [1]

$M_r(MgCO_3) = 24 + 12 + 48 = 84$ .
Moles $= 2.0 / 84 = 0.0238$ mol.
Answer: 0.0238 mol (or 0.024)

(b) [1]

Volume $= 0.05$ dm³.
Moles $HNO_3 = 1.0 \times 0.05 = 0.05$ mol.
Answer: 0.05 mol

Equation ratio $MgCO_3 : HNO_3$ is 1 : 2.
Moles $HNO_3$ needed for 0.0238 mol $MgCO_3 = 0.0238 \times 2 = 0.0476$ mol.
We have 0.05 mol $HNO_3$ .
$0.05 > 0.0476$ , so $HNO_3$ is in excess.
Answer: Nitric Acid ( $HNO_3$ )

(d) [3]

Limiting reactant is $MgCO_3$ (0.0238 mol).
Ratio $MgCO_3 : CO_2$ is 1 : 1.
Moles $CO_2$ expected $= 0.0238$ mol.
Theoretical Mass $CO_2 = 0.0238 \times 44 = 1.047$ g.
Percentage Yield $= (\text{Actual} / \text{Theoretical}) \times 100$ .
Yield $= (0.48 / 1.047) \times 100 = 45.8\%$ .
Answer: 45.8% (Accept 45-46%)

16. [1]

Carbon dioxide ( $CO_2$ ) is a gas and escapes into the atmosphere from the open beaker, causing a loss in mass.

17. [1]

Any one of:
- Incomplete reaction.
- Loss of product during transfer/filtration.
- Side reactions occurring.
- Impure reactants.

18. [2]

$M_r(H_2SO_4) = 2 + 32 + 64 = 98$ .
Concentration (g/dm³) $= \text{Molarity} \times M_r$ .
$0.5 \times 98 = 49$ g/dm³.
Answer: 49 g/dm³

19. [3]

Moles $NaOH = (20/1000) \times 0.1 = 0.002$ mol.
Ratio $NaOH : H_2SO_4$ is 2 : 1.
Moles $H_2SO_4 = 0.002 / 2 = 0.001$ mol.
Volume $H_2SO_4 = 25 \text{ cm}^3 = 0.025 \text{ dm}^3$ .
Concentration $H_2SO_4 = 0.001 / 0.025 = 0.04$ mol/dm³.
Answer: 0.04 mol/dm³

20. [1]

Percentage purity is the percentage by mass of the pure compound in a sample containing impurities.

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