Chemical Composition and Formulas

Overview

This lecture covers how to determine the percent composition, empirical formula, and molecular formula of compounds based on experimental mass measurements and chemical formulas.

Percent Composition

• Percent composition is the percent by mass of each element in a compound.
• To calculate percent composition: (mass of element / total mass of compound) × 100%.
• Example: A 12.04 g sample with 7.34 g C, 1.85 g H, and 2.85 g N is 61.0% C, 15.4% H, and 23.7% N.
• Percent composition can also be determined from a compound’s formula and atomic/molar masses.

Determining Percent Composition from Formula Mass

• Use the atomic masses and the chemical formula to calculate the mass of each element in one mole of the compound.
• Example: For NH₃, percent N = (14.01 g / 17.03 g) × 100% = 82.27%; percent H = (3.024 g / 17.03 g) × 100% = 17.76%.
• The sum of percentages for all elements in a compound should be 100%.

Determining Empirical Formulas

• Empirical formula shows the simplest whole-number ratio of atoms in a compound.
• Steps:
  1. Convert mass of each element to moles using atomic masses.
  2. Divide each mole value by the smallest number of moles.
  3. Multiply all subscripts to get whole numbers if needed.
• Example: For Fe (34.97 g) and O (15.03 g), empirical formula is Fe₂O₃.

Empirical Formulas from Percent Composition

• Assume 100 g sample; use percent as grams for each element.
• Convert grams to moles, then proceed as above to find empirical formula.
• Example: A gas with 27.29% C and 72.71% O yields the empirical formula CO₂.

Determination of Molecular Formulas

• Molecular formula gives the actual number of each type of atom in a molecule.
• Need empirical formula and molar mass/molecular mass.
• Formula: n = (molecular mass) / (empirical formula mass).
• Multiply each subscript in the empirical formula by n to get molecular formula.
• Example: Empirical formula CH₂O (mass 30 g/mol), molecular mass 180 g/mol, so molecular formula is C₆H₁₂O₆.

Key Terms & Definitions

• Percent Composition — Percent by mass of each element in a compound.
• Empirical Formula — Simplest whole-number ratio of atoms in a compound.
• Molecular Formula — Actual number of atoms of each element in a molecule.
• Molar Mass — Mass in grams of one mole of a substance.

Action Items / Next Steps

• Practice exercises: Calculate percent composition and empirical/molecular formulas from given data.
• Review procedures for converting mass to moles and determining simplest ratios.
• Prepare for questions requiring both experimental and formula-based calculations.