Determining the Empirical and Molecular Formulas from Elemental Percentages

Understanding how to determine the empirical and molecular formulas of compounds from their percentage composition is a fundamental skill in chemistry. This article explains the process with clear, step-by-step instructions and examples.

What is the Empirical Formula?

The empirical formula of a compound represents the simplest whole-number ratio of the elements in the compound. Let's explore how to find the empirical formula for a compound that is 94.1% oxygen and 5.90% hydrogen by mass.

Converting Percentages to Grams

Assuming a 100 g sample of the compound, the mass of each element can be calculated as:

O: 94.1 g H: 5.90 g

Converting Grams to Moles

Next, we convert the gram mass to moles using the molar masses of the elements:

Molecular mass of O 16.00 g/mol Molecular mass of H 1.01 g/mol

Calculations:

Moles of O: 94.1/16.00 5.88 mol Moles of H: 5.90/1.01 5.84 mol

Finding the Simplest Mole Ratio

Divide each by the smallest number of moles (5.84) to get the mole ratio:

O: 5.88/5.84 1.01 H: 5.84/5.84 1

The mole ratio of O to H is approximately 1:1.

Empirical Formula

The empirical formula is derived from the simplest whole-number ratio of elements:

Empirical formula: OH

Exploring Molecular Formula with Known Molar Mass

Suppose a compound has a molecular mass of 34 g/mol and contains 5.88% hydrogen and 94.12% oxygen by mass. We can determine its molecular formula through these steps:

Step 1: Calculate Molar Masses

Convert the masses to moles:

H: 5.88/1.01 5.82 mol O: 94.12/16.00 5.88 mol

Step 2: Formulate the Empirical Formula

The empirical formula for these masses is HO, with a formula mass of 17 g/mol.

Step 3: Determine the Molecular Formula

Divide the molecular mass by the empirical formula mass to find the ratio:

Molecular formula: H_2O_2 (hydrogen peroxide, molar mass 34 g/mol)

Thus, the molecular formula of the compound is H2O2, hydrogen peroxide.

Conclusion

By understanding the steps for determining both empirical and molecular formulas from the percentage composition of a compound, you can accurately describe the composition of unknown substances. These skills are crucial for chemists and essential for anyone studying chemistry or forensic science.