What is molecular weight (mass?

What is the difference between atomic weight and atomic mass and molar mass and molecular weight?

• Answer:

  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.