A Level Chemistry OCR Practice Exam

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What is the definition of relative atomic mass (Ar)?

  1. The weighted mean mass of an atom of an element compared to the mass of hydrogen

  2. The weighted mean mass of an atom of an element compared to one-twelfth of the mass of carbon-12

  3. The average mass of all isotopes of an element

  4. The mass of an atom of an isotope compared to the mass of oxygen

The correct answer is: The weighted mean mass of an atom of an element compared to one-twelfth of the mass of carbon-12

Relative atomic mass (Ar) is defined as the weighted mean mass of an atom of an element compared to one-twelfth of the mass of a carbon-12 atom. This definition establishes a standard point of reference, making carbon-12 the benchmark because it is a stable and common isotope. The term "weighted mean" indicates that the relative atomic mass takes into account the abundance of each isotope present in nature, thus providing a value that reflects the average mass of an atom of the element as it occurs in the environment. The use of one-twelfth of the mass of carbon-12 as the standard allows for consistent and accurate comparisons across different elements. This definition is crucial because it allows chemists to communicate the mass of atoms effectively and use these values in stoichiometric calculations, ensuring that chemical equations and reactions are balanced accurately.

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