Section 4 - Mass Percent Composition - Math Tutor DVD

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Overview of this Lesson

In this lesson, we explore the concept of mass percent composition in Chemistry. Mass percent composition is calculated by comparing the molar mass of an element in a compound to the total molar mass of the entire compound. The final calculation yields a percentage which is useful when comparing various compounds with one another.

For example, lets examine H2O. This molecule consisists of two different elements. We might be interested to calculate what the mass percent composition of Hydrogen is in this molecule because it gives us a relative indication of how much mass of Hydrogen is bound in the structure using a percentage.

To perform the calculation, we just find the molar mass of the hydrogen from the periodic table and multiply by two, because there are two atoms of hydrogen in this molecule. We then divide by the total molar mass of the entire molecule and multiply this result by 100 to arrive at the mass percent composition of Hydrogen in Water.

The formula for percent composition for Hydrogen in Water:

Mass Percent Composition of Hydrogen in H2O = (Molar Mass All Hydrogen in H2O)/(Molar mass of H2O molecule) * 100

We could also perform the exact same calculation for Oxygen in H2O:

Mass Percent Composition of Oxygen in H2O = (Molar Mass All Oxygen in H2O)/(Molar mass of H2O molecule) * 100

When we perform these calculations for Water we get the following results:

Mass Percent Composition of Hydrogen in Water = 11.1%Mass Percent Composition of Oxygen in Water = 88.9%

Notice that these two percentages must add to 100% because these are the only two elements in this compound! In this video lesson, we will explore these concepts using numerous example problems to demonstrate how to calculate mass percent composition for the student.