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

Molar mass represents the mass of one mole of a substance, which is commonly measured in grams per mole (g/mol). This value is crucial in stoichiometry when performing calculations involving moles in chemical reactions and determining how much of a substance is needed or produced.

To understand why this definition is preferable, consider that the molar mass reflects the sum of the atomic masses of all the atoms in a molecule. For example, if you have glucose (C6H12O6), its molar mass can be calculated by adding the molar masses of carbon, hydrogen, and oxygen based on their respective quantities in the molecule.

This concept contrasts with the other options. The weight of one mole of atoms in any element refers specifically to elemental substances and does not account for compounds. The average mass of an element's isotopes pertains to atomic mass but does not adequately define molar mass in the context of compounds. Lastly, the volume occupied by one mole of gas at standard temperature and pressure relates to the ideal gas law but does not address the concept of molar mass, which focuses on mass rather than volume.

In summary, defining molar mass as the mass of one mole of a substance provides a clear and comprehensive understanding that applies