A Level Chemistry OCR Practice Exam

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How is relative formula mass defined?

The mass of a molecule compared to the mass of oxygen-16
The weighted mean mass of a formula unit compared to one-twelfth of carbon-12
The mass of an atom in its gaseous state
The average mass of all atoms in a chemical reaction

The correct answer is: The weighted mean mass of a formula unit compared to one-twelfth of carbon-12

The relative formula mass is best defined as the weighted mean mass of a formula unit compared to one-twelfth of the mass of carbon-12. This definition emphasizes that the relative formula mass takes into account the composition of a substance, commonly an ionic compound or a molecular compound, by considering the individual atomic masses of the elements in its empirical formula. Each atom's contribution is weighted according to its abundance in that formula unit, leading to a collective average mass that reflects the overall mass of that specific formula unit. This concept is rooted in the definition of the atomic mass unit (amu), where the carbon-12 isotope is set as a standard reference point, making it a universally accepted method for expressing relative masses of different elements and compounds. Hence, this allows chemists to effectively compare different substances and their masses in reactions and laboratory scenarios.