I'm a bit confused. I've seen both "molar mass" and "atomic weight" listed as 15.999 for oxygen, but I thought they were two different things. Can you explain why both values are listed as 15.999 and how they differ from each other?
Molar mass and atomic weight are related, but they are not the same thing. Molar mass is the mass of one mole of a substance and is expressed in grams per mole (g/mol). Atomic weight, also known as atomic mass, is the average mass of the atoms of an element and is expressed in atomic mass units (amu).

In the case of oxygen, the molar mass is 15.999 g/mol, which is the mass of one mole of oxygen atoms or molecules. The atomic weight of oxygen is also 15.999 amu, which is the average mass of oxygen atoms, taking into account the fact that there are two stable isotopes of oxygen (oxygen-16 and oxygen-18) with different masses and abundances. The atomic weight is calculated as a weighted average of the masses of all the isotopes of the element, taking into account their relative abundances.

In general, the atomic weight of an element is a more precise and accurate measure of the mass of the element, since it takes into account the different isotopes that exist. However, the molar mass is a more practical measure for many purposes, since it allows you to easily convert between mass and moles of a substance.