When people say that the relative atomic mass is based on C-12, they are referring to the scale used to measure and compare the masses of atoms. The scale is called the atomic mass unit (amu) or unified atomic mass unit (u). It is a unit that is used to express the relative masses of atoms and molecules.
The atomic mass unit is defined by assigning a mass of exactly 12 atomic mass units to the carbon-12 isotope (C-12), which is one of the stable isotopes of carbon. This means that one atom of carbon-12 is assigned a mass of exactly 12 amu.
Using this standard, the relative atomic masses of other elements are determined by comparing their masses to the mass of a carbon-12 atom. For example, oxygen is assigned a relative atomic mass of approximately 16 amu. This means that an oxygen atom is about 16 times as massive as a carbon-12 atom.
However, it's important to note that this is a relative scale and not an absolute measure of mass. The actual mass of an oxygen atom is not exactly 16 times the mass of a carbon-12 atom. The relative atomic mass is an average value that takes into account the different isotopes of an element and their relative abundance in nature.
In reality, oxygen consists of several isotopes, including oxygen-16, oxygen-17, and oxygen-18, which have slightly different masses. The relative atomic mass of 16 amu for oxygen represents an average of these isotopes, weighted by their abundance.