A Level Chemistry OCR Practice Exam 2025 - Free Chemistry Practice Questions and Study Guide

The Avogadro constant, denoted as \( N_A \), is defined as the number of particles, typically atoms or molecules, in one mole of a substance. The value of the Avogadro constant is approximately \( 6.02 \times 10^{23} \) mol\(^{-1}\). This constant serves as a bridge between the macroscopic world of grams and liters and the microscopic world of atoms and molecules, allowing chemists to count particles by weighing them.

This concept is crucial in stoichiometry, enabling calculations involving moles of substances in reactions. For instance, when determining how many molecules are present in a certain mass of a compound, using the Avogadro constant facilitates these conversions.

The other options do not accurately define the Avogadro constant; for example, the number of protons in an atom pertains to the atomic number and is different for each element. Similarly, the number of molecules in a liter of gas depends on conditions such as temperature and pressure, which are described by the ideal gas law, rather than a fixed value like the Avogadro constant. Atomic mass relates to the mass of a single atom and is not directly related to the count of particles in a mole. Therefore, option B