A Level Chemistry OCR Practice Exam

Relative molecular mass (Mr) is defined as the weighted mean mass of a molecule compared to one-twelfth of the mass of carbon-12. This definition is crucial because it establishes a standard measurement that allows chemists to compare the masses of different molecules relative to a fixed point, which is carbon-12.

This definition also incorporates the concept of abundance for isotopes, as the weighted mean involves considering the distribution of the isotopes present naturally in the sample. By using carbon-12 as the standard, relative molecular mass provides a consistent and universal framework for expressing the masses of molecules, which is vital in chemical calculations, especially in stoichiometry.

The other options do not accurately reflect the standard definition. For instance, comparing the mass of a molecule to oxygen-16 or the mass of an ion to carbon-12 does not align with how relative molecular mass is conventionally defined. Additionally, stating that Mr is the average mass of all types of molecules fails to specify the comparative nature of relative molecular mass and neglects the reference to carbon-12, which is a key component of its definition.