Question:   What is the difference between atomic weight and molecular weight?

Answer:

The atomic weight corresponds to the weight of an element, while the molecular weight corresponds to the weight of a chemical compound.

Atomic weight is measured in units of atomic mass units (amu) when referring to single atoms, or in grams/mole when referring to moles of an element.  The atomic weight of an element is equal to the number of protons plus the number of neutrons.

Molecular weight (also called "molar mass" or "gram formula mass") is measured in units of grams/mole when referring to moles of an element.  The molecular weight of a compound is found by adding the atomic weights of all of the atoms in the element.  Water, for example, has a molecular weight of 18 grams/mole, as there are two hydrogen atoms that each weigh 1 gram/mole and one oxygen atom that weighs 16 grams/mole.