Early Chemical Discoveries and the Atomic Theory

The total mass of substances present after a chemical reaction is the same as the total mass of substances before the reaction. 

The apparent product of the combustion of the match— the ash— weighs less than the match. The product of the combustion of the magnesium ribbon (the smoke ) weighs more than the ribbon. Actually, in each case, the total mass remains unchanged. To understand this, you have to know that oxygen gas enters into both combustions and that water and carbon dioxide are also products of the combustion of the match. 

455 g sample of magnesium is allowed to burn in 2.315 g of oxygen gas. The sole product is magnesium oxide. After the reaction, no magnesium remains and the mass of unreacted oxygen is 2.015 g. What mass of magnesium oxide is produced  

The total mass is unchanged. The total mass is the sum of the masses of the substances present initially. The mass of magnesium oxide is the total mass minus the mass of unreacted oxygen. 

determine the total mass before the reaction. 

---

Early Chemical Discoveries and the Atomic Theory— Modern chemistry began with eighteenth-century discoveries leading to the formulation of two basic laws of chemical combination, the law of conservation of mass and the law of constant composition (definite proportions). These discoveries led to Dalton s atomic theory— that matter is composed of indestructible particles called atoms, that the atoms of an element are identical to one another but different from atoms of all other elements, and that chemical compounds are combinations of atoms of different elements. Based on this theory, Dalton proposed still another law of chemical combination, the law of multiple proportions. 

---