define relative masses of atoms and molecules based on carbon-12

The relative atomic mass (Ar) and relative molecular mass (Mr) are important concepts in chemistry. They are based on a scale in which the atoms of the isotope carbon-12 (12C) are assigned a mass of exactly 12 atomic mass units (amu).

1. Relative Atomic Mass (Ar): This is a dimensionless quantity that is the ratio of the average mass of atoms of an element (from a given sample or source) to 1/12 of the mass of an atom of carbon-12. It takes into account the naturally occurring isotopes of the element.

2. Relative Molecular Mass (Mr): This is also a dimensionless quantity that is the ratio of the average mass of a molecule (from a given sample or source) to 1/12 of the mass of an atom of carbon-12. It is calculated as the sum of the relative atomic masses of the atoms in a molecule.

For example, the relative atomic mass of hydrogen (H) is approximately 1 because a hydrogen atom has about 1/12 the mass of a carbon-12 atom. Similarly, the relative molecular mass of water (H2O) is approximately 18 (since it is composed of 2 hydrogen atoms and 1 oxygen atom, and the relative atomic masses of hydrogen and oxygen are approximately 1 and 16 respectively).