The purpose of comparing the average atomic mass of an element with 1/12 of the mass of C-12 is to establish a standard reference point for measuring atomic masses. This reference point is used to define the atomic mass unit (amu), which is a unit of mass commonly used in chemistry and atomic physics.
While it is true that the mass of C-12 is 12 atomic mass units, and 1/12 of that mass is 1 atomic mass unit, the reason for using this comparison is historical and practical. In the early development of atomic theory, scientists needed a consistent and agreed-upon standard for measuring atomic masses. They chose the isotope carbon-12 (C-12) as the reference because it is abundant, relatively stable, and has a whole number mass.
By comparing the masses of other elements or isotopes to 1/12 of the mass of C-12, scientists can determine their relative masses. The average atomic mass of an element takes into account the abundance of each naturally occurring isotope of that element. Since different isotopes of an element have different masses, the average atomic mass reflects the weighted average of all the isotopes' masses based on their abundance.
Using the carbon-12 standard allows for consistent and standardized measurements across the scientific community. It facilitates accurate comparison and calculation of atomic masses, which is essential for various applications, including chemical reactions, stoichiometry, and understanding the properties of different elements and compounds.