Pure Substances and Percent Composition

Ever wonder what makes water always water, no matter where you find it? It's because pure substances have a fixed composition - they're made of only one kind of particle (atoms, compounds, or molecules) in a consistent ratio.

Think of pure substances like a perfect recipe that never changes. Water (H₂O) always has a 1:2 ratio of oxygen to hydrogen atoms, which translates to a mass ratio of 16 grams of oxygen to 2 grams of hydrogen. This consistency is what defines pure substances.

You can calculate the percentage of each element in a compound using this formula:

% Mass = (Total Mass of Element ÷ Total Mass of Compound) × 100

For example, in water (H₂O), oxygen makes up 89% of the mass (16g ÷ 18g × 100), while hydrogen accounts for only 11% (2g ÷ 18g × 100).

💡 Quick Tip: When analyzing unknown compounds, remember that different chemical formulas can have the same percent composition if they maintain the same ratio of elements. For instance, N₂O₄, N₃O₆, and N₄O₈ all have approximately 30.4% nitrogen and 69.6% oxygen because they all have the same 1:2 ratio of N to O.

Molecular vs. Empirical Formulas

Did you know that the formula you see for a compound might not show its simplest ratio? A molecular formula shows the actual number of atoms of each element in a compound, while an empirical formula shows the lowest whole-number ratio of those atoms.

Take butane (C₄H₁₀) for example - its molecular formula tells us exactly what's in one molecule, but its empirical formula (C₂H₅) shows us the simplest ratio of carbon to hydrogen. Sometimes they're identical, like in formaldehyde (CH₂O), where the formula can't be simplified further.

When given percent composition data, you can determine a compound's empirical formula in four steps: First, assume 100g of the compound and convert the percentages to grams, then to moles. Second, write the formula using these mole values. Third, divide all by the smallest mole value to find the simplest ratio. Finally, if needed, multiply by a whole number to get values close to integers.

🔍 Important Note: If you know both the empirical formula and the molecular mass, you can find the molecular formula by comparing masses. The ratio between the molecular mass and empirical formula mass tells you how many empirical formula units make up one molecule.