The mass of a chemical compound is the sum of the mass of each of the elements in the compound. The molar mass of each chemical element is known.
Carbon dioxide has the chemical formula CO2 and each molecule has one atom of carbon and 2 atoms of oxygen. The molar mass of carbon is 12 g/mole and the molar mass of oxygen is 16 g/mole. The molar mass of carbon dioxide is 44 g/mole. The mass of 35.3 moles is 35.3*44 = 1553.2 g
The number of grams of carbon dioxide in a sample that has 35.3 moles of the compound is 1553.2 g