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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. The empirical formula of a hydrocarbon is CH2 and its molar mass is 56 g mol-1. What is its molecular formula? [RAM: C=12, H=1]

  1. C2H4
  2. C3H6
  3. C4H8
  4. C5H10
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Answer: C

Empirical formula mass = 12 + 2(1) = 14. n = 5614 = 4. Molecular formula = (CH2)4 = C4H8.

2. 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.

3. 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.

4. 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.

5. 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.

6. How many oxygen molecules are contained in 32 g of oxygen gas, O2? [RAM: O=16; Avogadro constant = 6.02 × 1023 mol-1]

  1. 1.204 × 1024
  2. 6.02 × 1023
  3. 3.01 × 1023
  4. 6.02 × 1022
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Answer: B

Molar mass O2 = 32 g mol-1. Moles O2 = 3232 = 1.0 mol. Molecules = 1.0 × 6.02 × 1023 = 6.02 × 1023.

7. What is the number of moles of nitrate ions, NO3-, in 0.2 mol of aluminium nitrate, Al(NO3)3?

  1. 0.2 mol
  2. 0.4 mol
  3. 0.6 mol
  4. 0.8 mol
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Answer: C

1 mol Al(NO3)3 dissociates to produce 3 mol NO3- ions. Moles NO3- = 0.2 × 3 = 0.6 mol.

8. What is the volume occupied by 0.2 mol of carbon dioxide gas at room conditions? [Molar volume = 24 dm3 mol-1 at room conditions]

  1. 4.8 dm3
  2. 4.48 dm3
  3. 0.48 dm3
  4. 480 cm3
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Answer: A

Volume = moles × molar volume = 0.2 mol × 24 dm3 mol-1 = 4.8 dm3.

9. 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.

10. 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.

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