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Quiz Chapter 3: The Mole Concept, Chemical Formulae and Equations

10 questions · Form 4 Chemistry Bab 3: The Mole Concept, Chemical Formulae and Equations

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Full Question List & Answer Key

Prefer reading to quizzing? All 10 questions are listed below with the answer and explanation under each one.

1. Which gas occupies a volume of 1.2 dm3 at room conditions? [RAM: H=1, C=12, N=14, O=16; Molar volume at room conditions = 24 dm3 mol-1]

  1. 1.6 g of O2
  2. 0.8 g of CH4
  3. 1.4 g of N2
  4. 2.2 g of CO2
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Answer: A

Required moles = 1.224 = 0.05 mol. Mass O2 = 0.05 × 32 = 1.6 g (matches 1.6g O2).

2. Which isotope is used as the international standard reference for relative atomic mass scale?

  1. Hydrogen-1
  2. Carbon-12
  3. Oxygen-16
  4. Chlorine-35
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Answer: B

Carbon-12 is assigned a relative atomic mass of exactly 12.000 and serves as the primary standard.

3. A compound contains 80% carbon and 20% hydrogen by mass. What is its empirical formula? [RAM: C=12, H=1]

  1. CH2
  2. CH4
  3. CH3
  4. C2H5
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Answer: C

Moles C = 8012 = 6.67. Moles H = 201 = 20. Ratio C:H = 6.676.67 : 206.67 = 1 : 3. empirical formula = CH3.

4. How many atoms are present in 0.5 moles of helium gas? [Avogadro constant, Na = 6.02 × 1023 mol-1]

  1. 1.204 × 1024
  2. 3.01 × 1023
  3. 6.02 × 1023
  4. 3.01 × 1024
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Answer: B

Helium is a monatomic gas. Number of atoms = 0.5 × 6.02 × 1023 = 3.01 × 1023.

5. A container contains 3.01 × 1023 molecules of water, H2O. What is the mass of the water sample? [RAM: H=1, O=16; Avogadro constant = 6.02 × 1023 mol-1]

  1. 18 g
  2. 9 g
  3. 36 g
  4. 4.5 g
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Answer: B

Moles H2O = 3.01 × 10236.02 × 1023 = 0.5 mol. Molar mass = 18 g mol-1. Mass = 0.5 × 18 = 9 g.

6. Which method is most appropriate to experimentally determine the empirical formula of copper(II) oxide?

  1. Heating copper turnings in a closed crucible
  2. Reducing copper(II) oxide using dry hydrogen gas
  3. Precipitating copper oxide with sodium hydroxide
  4. Electrolyzing copper(II) sulfate solution
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Answer: B

Copper is less reactive than hydrogen, so dry hydrogen gas reduces heated copper(II) oxide to copper metal.

7. Calculate the mass of 0.5 moles of calcium carbonate, CaCO3. [RAM: Ca=40, C=12, O=16]

  1. 100 g
  2. 50 g
  3. 25 g
  4. 150 g
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Answer: B

Molar mass CaCO3 = 40 + 12 + (3 × 16) = 100 g mol-1. Mass = 0.5 × 100 = 50 g.

8. In determining the empirical formula of magnesium oxide, why is the crucible lid lifted periodically during heating?

  1. To allow oxygen from the air to react with magnesium while preventing loss of oxide smoke
  2. To release excess heat and prevent the crucible from cracking
  3. To allow water vapor to escape from the reaction mixture
  4. To observe the color change of magnesium ribbon
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Answer: A

Lifting the lid admits oxygen for complete reaction, while closing it rapidly catches escaping white MgO smoke particles.

9. Which definition correctly describes the empirical formula of a chemical compound?

  1. The formula showing the total mass of each element in 100g of compound
  2. The formula showing the actual number of atoms of each element in a molecule
  3. The formula showing the simplest whole-number ratio of atoms of each element
  4. The structural layout of bonds between atoms in a molecule
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Answer: C

Empirical formula is strictly defined as the simplest whole-number atomic ratio of constituent elements.

10. What mass of magnesium is required to produce 11.2 dm3 of hydrogen gas at STP when reacted with excess hydrochloric acid? [Mg + 2HCl -> MgCl2 + H2] [RAM: Mg=24; Molar volume at STP = 22.4 dm3 mol-1]

  1. 12 g
  2. 24 g
  3. 6 g
  4. 48 g
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Answer: A

Moles H2 = 11.222.4 = 0.5 mol. Mole ratio Mg:H2 = 1:1, so moles Mg = 0.5 mol. Mass Mg = 0.5 × 24 = 12 g.

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