Let’s begin with an example:
Suppose I have a glucose molecule. (C6H1206) What percentage of the molecule’s mass is carbon?
The first step is to identify the atomic weights of the various elements in glucose. With the help of a periodic table of elements, we know that carbon‘s atomic weight is roughly 12.011 atomic mass units (amu). Hydrogen‘s atomic weight is roughly 1.008 amu. Oxygen has an atomic weight of 15.999 amu.
The second step is to multiply the element’s atomic weight by the number of atoms in the molecule, and add the new atomic weights together. Let’s take carbon as an example. There are 6 carbon atoms in glucose. Therefore, we multiply 6 by 12.011. We get 72.066 amu. See if you can find the atomic weight of glucose using this strategy.
So you should have gotten about 180.156 amu as the mass for the entire molecule. In order to solve the original question you divide 72.066 (carbon’s mass) by 180.156. The answer is 4/10. Therefore, 40 percent of a glucose molecule’s mass is carbon. We can apply this on a larger scale also. If I had 10 grams of glucose, I could expand 4 grams to be pure carbon.
Using this knowledge, we can predict how matter will change during a chemical reaction. We can also figure out the amount of a substance that will be created during processes such as combustion. This is only the tip of the iceberg. Scientists can deduce more about chemical processes using units such as the mole and employing empirical formulas.