Which observation illustrates the law of conservation of mass? The C:O mass ratio of a particular compound is the same, regardless of the size or source of the sample. / When 3 g of carbon reacts with 8 g of oxygen, 11 g of carbon dioxide is produced. / The C:O mass ratio of one compound is exactly double that of another compound./ Burning 10 g of propane produces twice as much carbon dioxide as burning 5 g of propane.