LAW OF CONSERVATION OF MASS BASICS AND TUTORIALS

The process of combustion burning is so familiar that it is hard to realize what a difficult riddle it posed for early scientists.

In 1774, Antoine Lavoisier (1743 1794) performed an experiment in which he heated a sealed glass vessel containing a sample of tin and some air. He found that the mass before heating and after heating were the same.


(glass vessel + tin calx + remaining air) = (glass vessel + tin + air)


Through further experiments, he showed that the product of the reaction, tin calx (tin oxide), consisted of the original tin together with a portion of the air. Experiments like this proved to Lavoisier that oxygen from air is essential to combustion, and also led him to formulate the law of conservation of mass:

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


Applying the Law of Conservation of Mass

A 0.455 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?

Analyze
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.

Solve

First, determine the total mass before the reaction. The total mass after the reaction is the same as before the reaction. Solve for the mass of magnesium oxide.


= 0.755 g magnesium oxide after reaction
- 2.015 g oxygen after reaction
? g magnesium oxide after reaction = 2.770 g mass after reaction
+ 2.015 g oxygen after reaction
2.770 g mass after reaction = ? g magnesium oxide after reaction
= 2.770 g mass before reaction
mass before reaction = 0.455 g magnesium + 2.315 g oxygen


Assess
Here is another approach. The mass of oxygen that reacted is 2.315 g 2.015 g 0.300 g. Thus, 0.300 g oxygen combined with 0.455 g magnesium to give 0.300 g 0.455 g 0.755 g magnesium oxide.