Relative atomic mass

Relative atomic mass
Relative atomic mass is abbreviated to Ar.
It is the mean mass of an atom relative to 1/12th the mass of an atom of 126C.
By definition, Ar(C) = 12 exactly. Use the Periodic Table to find the Ar values of other elements.
Relative formula mass
Relative formula mass is abbreviated to Mr. It is often also called relative molecular mass.
It is the mean mass of a molecule relative to 1/12th the mass of an atom of 126C.
To find the Mr of a substance, do these things: