Which of the following best defines a mole in chemistry?

• A. A mole is the number of atoms in one molecule of water.
• B. A mole is the mass of a substance equal to one gram.
• C. A mole is the amount of substance that contains exactly 6.022 × 10^{23} elementary entities.
• D. A mole is the volume occupied by a gas at standard temperature and pressure.

Answer: (C)

Verified step by step guidance

1. Understand that a mole is a fundamental concept in chemistry used to count particles such as atoms, molecules, or ions, similar to how a dozen counts 12 items.
2. Recall that the mole is defined by a specific number called Avogadro's number, which is exactly \$6.022 \times 10^{23}$ elementary entities (atoms, molecules, ions, etc.).
3. Recognize that the mole does not refer to the number of atoms in a single molecule (like water), nor is it defined by mass (such as one gram) or volume (such as gas volume at STP).
4. Identify that the correct definition of a mole is the amount of substance containing exactly \$6.022 \times 10^{23}$ elementary entities, making it a counting unit for particles.
5. Use this definition to distinguish the mole from other concepts like mass or volume, which are related but not the defining characteristic of a mole.