How do astronomers measure the amount of dark matter in the universe

Iron pyrite (fes2) is the form in which much of the sulfur exists in coal. in the combustion of coal, oxygen reacts with iron pyrite to produce iron(iii) oxide and sulfur dioxide, which is a major source of air pollution and a substantial contributor to acid rain. what mass of fe2o3 is produced from 84 l of oxygen at 3.41 atm and 148◦c with an excess of iron pyrite? answer in units of g.