What is the difference between atomic weight and atomic mass and molar mass and molecular weight?
The atomic mass is the mass of an atom at rest. This
refers to one atom, and is typically measured in so-called atomic
mass units, and describes a single isotope of that element. The
atomic weight is the abundance-weighted average mass of an
element (an average mass of all the isotopes that exist for that
element, weighted by how abundant each isotope is in nature).
Atomic weight is commonly reported in atomic mass units, as well as
in grams per mole. The molar mass is the mass of one mole of
something, and can refer to both a single element as well as
molecules and other compounds. The molecular weight is
essentially the same thing as the molar mass except that, as the
name implies, it refers to molecules rather than just elements. The
molar mass and molecular weight is typically given in units of
grams per mole.
The molecular weight of a compound is found by adding the atomic
weights of all of the atoms in the compound. Water, for example,
has a molecular weight of 18.0152 grams/mole, as there are two
hydrogen atoms that each have an atomic weight of 1.00079 g/mole
and one oxygen atom with an atomic weight 15.9994 grams/mole.