CXC - Caribbean Examinations Council Chemistry: The Mole Concept Questions and Answers

Question 1

What does Avogadro's constant represent?

Accepted Answer

The number of particles (atoms, molecules, or ions) in one mole of a substance.

Answer

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)

Accepted Answer

0.5 moles

Answer

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)?

Accepted Answer

24.0 dm³

Answer

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)

Accepted Answer

128 g

Answer

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?

Accepted Answer

The amount of substance that contains the same number of particles as there are atoms in exactly 12 g of carbon-12.

Answer

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.

CXC - Caribbean Examinations Council Practice Test

CXC - Caribbean Examinations Council Practice Test

CXC - Caribbean Examinations Council Chemistry: The Mole Concept Questions and Answers