When chemicals react, atoms inside them swap places to make new compounds, which are the products of the reaction. The atoms are the same as before in quality and number, but they have been rearranged. The overall mass stays the same. It is possible to predict the yield of a reaction using the chemical equation.
Example: Calcium carbonate (CaCO3) is thermally decomposed into calcium oxide (CaO) and carbon dioxide (CO2). The symbol equation is as follows:
CaCO3 CaO + CO2
How much carbon dioxide is made when 1000g of calcium carbonate decomposes?
Answer: In order to calculate this without knowing at least the amount of calcium oxide made from 1000g of calcium carbonate using conservation of mass, we need to calculate the relative formula masses of the compounds involved. In this case, calcium carbonate and carbon dioxide.
Calcium carbonate has 1 atom of calcium, 1 atom of carbon and 3 atoms of oxygen, and we need to add the relative atomic masses of all those atoms to find the relative formula mass, so:
relative formula mass of calcium carbonate: 40 + 12 + 16 + 16 + 16 = 100
Carbon dioxide has 1 atom of carbon and 2 atoms of oxygen, so:
relative formula mass of carbon dioxide: 12 + 16 + 16 = 44
Using the relative formula masses and the equation for the reaction, we know that:
100g of calcium carbonate gives 44g of carbon dioxide
1000g is 10 times more than 100g, so 1000g of calcium carbonate gives 10 x 44 = 440 g of carbon dioxide.