* Multiple Compounds Can Have the Same Percent Composition: Consider the following examples:
* Formaldehyde (CH₂O) has a percent composition of 40% carbon, 6.7% hydrogen, and 53.3% oxygen.
* Acetic Acid (C₂H₄O₂) also has a percent composition of 40% carbon, 6.7% hydrogen, and 53.3% oxygen.
Even though both have the same percentage composition, they have very different molecular formulas.
* Percent Composition Represents Ratios, Not Exact Numbers: Percent composition tells us the ratio of elements in the compound. For example, a compound with a percent composition of 85.7% carbon and 14.3% hydrogen could be:
* CH₂ (simplest ratio)
* C₂H₄
* C₃H₆, and so on.
To determine the molecular formula, you need additional information:
* Molar Mass (Molecular Weight): Knowing the molar mass of the compound allows you to calculate the exact number of each element present in the molecule.
* Empirical Formula: This formula represents the simplest whole-number ratio of elements in the compound. You can determine the molecular formula from the empirical formula and the molar mass.
In Summary
Percent composition is a useful tool for understanding the relative proportions of elements in a compound, but it's insufficient to determine the actual molecular formula. You need additional information, such as molar mass, to determine the exact number of atoms of each element in the molecule.
