Which of the following statements best describes a mole in chemistry?

• A. A mole is the volume occupied by 1 gram of a substance at standard temperature and pressure.
• B. A mole is the number of atoms in 1 gram of hydrogen.
• C. A mole is the amount of substance that contains exactly 6.022 × 10^{23} elementary entities.
• D. A mole is the mass of a substance equal to its atomic number in grams.

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.
2. Recall that the mole is defined as the amount of substance containing exactly 6.022 × 10^{23} elementary entities, which is known as Avogadro's number.
3. Recognize that the mole is not related to volume or mass directly, but rather to the number of particles, regardless of the substance's physical state or mass.
4. Note that the volume occupied by a substance depends on its state and conditions (like temperature and pressure), so it cannot define a mole.
5. Understand that the mass of a substance equal to its atomic number in grams is incorrect because the mole relates to atomic or molecular mass (in atomic mass units) converted to grams, not the atomic number.