Percentage Composition

Percentage composition is the measure of how much each chemical element contributes to the total mass of a compound, rather than the simple number of its atoms. It reveals the exact percentage by weight of each substance present within a molecule.

Atom count versus mass

When examining a chemical formula such as water, it is easy to assume that the element with more atoms makes up the larger share of the substance. Water contains two hydrogen atoms joined to a single oxygen atom, which might make it seem like water is mostly hydrogen. However, atoms vary greatly in size and weight, meaning the number of atoms alone cannot determine whether an element forms more or less of a compound.

The fruit box comparison

A simple way to understand this concept is to look at a fruit box containing two apples and one pineapple. Counting the pieces gives two apples and only one pineapple, but the pineapple is much larger and heavier. Because of its greater size and weight, the single pineapple represents most of the total mass in the box. In the same way, an oxygen atom is much larger and heavier than hydrogen atoms.

Percentage composition of water

• Hydrogen atoms have very little mass, so the two hydrogen atoms in water account for only 11.19 percent of its total mass.
• Oxygen atoms are significantly heavier, so the single oxygen atom makes up 88.8 percent of the mass of water.
• Measuring by mass rather than atom count explains which element truly dominates a compound.

Distribution of oxygen on Earth

Oxygen is the most abundant element on Earth, but only 21 percent of it exists in the air as a gas. A large portion of the world's oxygen remains chemically combined in water and solid ground. In the past, oxygen reacted with silicon in the sea to turn into soil and build large landmasses, such as the ancient continent Pangaea. Because it is bound into both soil and water, oxygen is widely available across the planet.