Overview
This lecture explains how to calculate the formula mass for covalent and ionic compounds by summing the average atomic masses of the atoms in their formulas.
Formula Mass Basics
- Formula mass is determined by adding the average atomic masses of all atoms in a chemical formula.
- For covalent compounds, the formula mass is the molecular mass and represents a single molecule.
- For ionic compounds, the formula mass represents the sum for the formula unit, not a discrete molecule.
Calculating Molecular Mass for Covalent Compounds
- The molecular mass of chloroform (CHCl₃) is 119.37 amu, calculated by adding masses for 1C, 1H, and 3Cl atoms.
- For aspirin (C₉H₈O₄), the molecular mass is 180.15 amu, summing up 9C, 8H, and 4O atoms.
- Example: Ibuprofen (C₁₃H₁₈O₂) has a molecular mass of 206.27 amu.
- Practice: Acetaminophen (C₈H₉NO₂) has a molecular mass of 151.16 amu.
Calculating Formula Mass for Ionic Compounds
- The formula mass of sodium chloride (NaCl) is 58.44 amu, computed as 1Na + 1Cl.
- Use the average masses of neutral atoms when calculating ionic compound formula masses; electron differences are negligible.
- Example: Aluminum sulfate [Al₂(SO₄)₃] has a formula mass of 342.14 amu (2Al, 3S, 12O).
- Practice: Calcium phosphate [Ca₃(PO₄)₂] has a formula mass of 310.18 amu.
Key Terms & Definitions
- Formula Mass — The sum of average atomic masses of all atoms in a compound’s formula.
- Molecular Mass — Formula mass for a covalent molecule; represents a single molecule’s mass.
- Ionic Compound — Substance comprised of positive and negative ions in specific ratios.
- Atomic Mass Unit (amu) — Standard unit for expressing atomic and molecular masses.
Action Items / Next Steps
- Practice calculating formula and molecular masses using various chemical formulas.
- Review and memorize average atomic masses for common elements.
- Complete any assigned textbook problems on formula mass calculations.