Percent composition is important because it helps one to know the chemical composition of certain substances. Percent composition is used to calculate the percentage of an element in a mixture. One can also derive an empirical formula from percent composition. The empirical formula derived from percent composition can help one find the actual molecular weight.
To calculate percent composition, one needs to find the weight of each atom present, then get the percentages of the total weights of the molecules in the mixture. The formula used to calculate percent composition of compounds is based on the weights of each constituent element.
An example that illustrates percent composition is calculating the percentage of oxygen in air, which is 21 percent. Any percentage higher than 21 can result in oxygen poisoning if breathed in for a prolonged time.
Solutions have different percent compositions of water and the element mixed with the water. For example, one can determine the importance of percent composition of water in hydrate compounds by knowing the percentage of individual elements. When ionic compounds are crystallized in water, they take up a definite water proportion as a part of their structure. The compound containing the water is called water of hydration. If one heats the hydrate to a high temperature, the water of hydration separates from the solution through evaporation, leaving behind anhydrous salt.