Percentage Composition of Compounds

Ever wonder what makes up the chemicals around you? Percentage composition tells you exactly how much of each element is in a compound. It's calculated by dividing the mass contribution of an element by the total mass of the compound, then multiplying by 100.

The formula is simple: Mass % of element = (number of moles × molar mass of element ÷ molar mass of compound) × 100

Let's see this in action with iron(III) oxide (Fe₂O₃). This compound has a molar mass of 159.79 g/mol. When we calculate iron's percentage, we get: (2 mol Fe × 55.85 g/mol) ÷ 159.79 g/mol × 100 = 69.94% Fe

Similarly, for oxygen: (3 mol O × 16.00 g/mol) ÷ 159.79 g/mol × 100 = 30.06% O

Try This! Calculate the percentage of nitrogen in ammonium nitrate (NH₄NO₃). The answer is 35.02% - can you confirm this using the formula?

We can also work backward! If we know the percentage composition (like 43.7% P and 56.3% O), we can find the empirical formula. First, convert percentages to moles, then find the simplest whole-number ratio. For phosphorus and oxygen, this gives us P₂O₅.

You'll use these calculations often in chemistry, so practice until they become second nature!