sulfur trioxide decomposes to sulfur dioxide and oxygen gas as shown, given a 240 g sample of sulfur trioxide how many grams of oxygen are produced, assuming the decomposition goes to completion

Because electrons have negligible mass, to account for the mass of the isotope, there must be 16 neutrons, 16 neutrally charged, massive, fundamental particles present in the sulfur nucleus. and thus finally, if we have the 32s2− ion, there are 18 electrons, 16 protons, and 16 neutrons.

It shows that the experiment is designed properly.