CXC - Caribbean Examinations Council Chemistry: The Mole Concept Questions and Answers — Questions and Answers
Question 1: What does Avogadro's constant represent?
- The volume occupied by one mole of any gas at standard temperature and pressure.
- The number of particles (atoms, molecules, or ions) in one mole of a substance. (Correct answer)
- The mass in grams of one mole of the carbon-12 isotope.
- The total number of protons and neutrons in the nucleus of a carbon-12 atom.
Correct answer: The number of particles (atoms, molecules, or ions) in one mole of a substance.
Avogadro's constant, approximately 6.02 x 10²³ mol⁻¹, is the number of constituent particles (such as atoms, molecules, or ions) that are contained in one mole of a substance. This value is fundamental to relating the macroscopic amount of a substance (in moles) to the microscopic number of particles.
Question 2: A student needs to calculate the number of moles in a 50 g sample of calcium carbonate (CaCO₃). What is the correct value? (Relative atomic masses: Ca = 40, C = 12, O = 16)
- 2.0 moles
- 1.0 mole
- 0.5 moles (Correct answer)
- 100 moles
Correct answer: 0.5 moles
First, calculate the molar mass of CaCO₃: (1 × Ca) + (1 × C) + (3 × O) = (1 × 40) + (1 × 12) + (3 × 16) = 40 + 12 + 48 = 100 g/mol. Then, use the formula: Moles = Mass / Molar Mass. Moles = 50 g / 100 g/mol = 0.5 moles.
Question 3: Which of the following describes the volume occupied by one mole of any gas at room temperature and pressure (RTP)?
- It depends on the molar mass of the gas.
- 22.4 dm³
- 6.02 x 10²³ dm³
- 24.0 dm³ (Correct answer)
Correct answer: 24.0 dm³
According to Avogadro's Law, one mole of any ideal gas will occupy the same volume under the same conditions of temperature and pressure. At Room Temperature and Pressure (RTP), which is defined as 25°C (298 K) and 1 atm, this volume is 24.0 dm³. The value 22.4 dm³ applies to Standard Temperature and Pressure (STP).
Question 4: What is the mass of 1.2 x 10²⁴ molecules of sulfur dioxide (SO₂)? (Relative atomic masses: S = 32, O = 16; Avogadro's constant = 6.0 x 10²³ mol⁻¹)
- 128 g (Correct answer)
- 64 g
- 32 g
- 256 g
Correct answer: 128 g
This is a two-step calculation. First, find the number of moles: Moles = Number of molecules / Avogadro's constant = (1.2 x 10²⁴) / (6.0 x 10²³) = 2 moles. Second, calculate the molar mass of SO₂: (1 × S) + (2 × O) = 32 + (2 × 16) = 64 g/mol. Finally, find the mass: Mass = Moles × Molar Mass = 2 mol × 64 g/mol = 128 g.
Question 5: Which of the following statements correctly defines the mole?
- The mass of a substance that contains 6.02 x 10²³ grams of particles.
- The amount of substance that contains the same number of particles as there are atoms in exactly 12 g of carbon-12. (Correct answer)
- The relative atomic mass of a substance expressed in grams.
- The volume of a substance that contains exactly 24 dm³ of particles at RTP.
Correct answer: The amount of substance that contains the same number of particles as there are atoms in exactly 12 g of carbon-12.
The mole is the SI unit for the amount of a substance. It is formally defined as the amount of substance that contains as many elementary entities (atoms, molecules, etc.) as there are atoms in exactly 12 grams of the carbon-12 isotope.
Question 6: A chemical reaction produces 4.8 dm³ of carbon dioxide gas (CO₂) at Room Temperature and Pressure (RTP). How many moles of CO₂ were produced? (Molar volume of a gas at RTP = 24.0 dm³/mol)
- 0.5 moles
- 5.0 moles
- 2.0 moles
- 0.2 moles (Correct answer)
Correct answer: 0.2 moles
The relationship between volume and moles for a gas at RTP is given by the formula: Moles = Volume / Molar Volume. Plugging in the given values: Moles = 4.8 dm³ / 24.0 dm³/mol = 0.2 moles.
CXC - Caribbean Examinations Council Chemistry: The Mole Concept Questions and Answers
What does Avogadro's constant represent?