Percentage Composition by Mass

Definition of Percentage Composition

Percentage composition by mass shows the proportion of each element in a compound expressed as a percentage of the total mass of that compound. It tells you how much of the compound’s mass comes from each element.

This is always given as % by mass, calculated from the total mass of the compound. It helps chemists understand the makeup of substances and compare different compounds.

Calculating Percentage Composition

To calculate the percentage composition of an element in a compound:

1. Find the relative atomic masses (Ar) of each element from the periodic table.
2. Calculate the relative formula mass (Mr) of the compound by adding the Ar values of all atoms in the formula.
3. Calculate the total mass of the element in the compound by multiplying its Ar by the number of atoms of that element in the formula.
4. Use the formula:

$\%\text{ of element} = \frac{\text{mass of element in compound}}{\text{total Mr of compound}} \times 100$

This gives the percentage by mass of that element in the compound.

For example, water (H₂O) has 2 hydrogen atoms and 1 oxygen atom. The Ar of hydrogen is approximately 1, and oxygen is approximately 16.

So, the total Mr of water is $2 \times 1 + 16 = 18$.

The mass of hydrogen in water is $2 \times 1 = 2$.

Therefore, the percentage of hydrogen in water is:

$$\frac{2}{18} \times 100 = 11.1\%$$

Similarly, the percentage of oxygen is:

$$\frac{16}{18} \times 100 = 88.9\%$$

Applications of Percentage Composition

Percentage composition is useful for:

• Determining purity: If a sample contains impurities, its percentage composition will differ from the pure compound’s theoretical values.
• Comparing compounds: Helps identify and compare substances based on their elemental makeup.
• Further calculations: It is the basis for calculating empirical formulas and molecular formulas (covered in other topics).

Example: Calculating Percentage Composition of Carbon in Carbon Dioxide (CO₂)

First, find the relative atomic masses: Ar(C) = 12, Ar(O) = 16.

Calculate the Mr of CO₂:

$$12 + (2 \times 16) = 12 + 32 = 44$$

Mass of carbon in CO₂ is 12.

Calculate percentage of carbon:

$$\frac{12}{44} \times 100 = 27.3\%$$

So, carbon makes up 27.3% of the mass of carbon dioxide.

Visualising Percentage Composition

A pie chart can help visualise the percentage composition of elements in a compound. For example, in water (H₂O), the pie chart would show a small slice for hydrogen (11.1%) and a large slice for oxygen (88.9%). This visual aid helps remember which elements contribute most to the compound’s mass.