Adding what two values gives the mass of an atom?

In the 1800s, one of the statements in john dalton's atomic theory was that atoms are indivisible. later experimental evidence led to the discovery of subatomic particles such as neutrons, electrons, and protons. what happened to the indivisible atom part of dalton's atomic theory, and why?

When balancing a chemical equation:

How many grams of oxygen gas are contained in a 15 l sample at 1.02 atm and 28°c? show your work.