Formula Mass: Molecular and Ionic Compounds

Molecular Mass (Covalent Compounds)

  • Definition: Formula mass = sum of average atomic masses of all atoms in the substance’s formula; for covalent substances, this is the molecular mass.
  • CHCl$_3$ example:
    • Formula: CHCl$_3$ (1 C, 1 H, 3 Cl)
    • Calculation: M<em>extCHCl</em>3=1×12.01+1×1.008+3×35.45=119.37 amuM<em>{ ext{CHCl}</em>3} = 1 \times 12.01 + 1 \times 1.008 + 3 \times 35.45 = 119.37 \text{ amu}
  • C$9$H$8$O$_4$ example:
    • Calculation: M<em>extC</em>9extH<em>8extO</em>4=9×12.01+8×1.008+4×16.00=180.15 amuM<em>{ ext{C}</em>9 ext{H}<em>8 ext{O}</em>4} = 9 \times 12.01 + 8 \times 1.008 + 4 \times 16.00 = 180.15 \text{ amu}

Formula Mass of Ionic Compounds

  • Ionic compounds consist of discrete cations and anions; formula mass is computed the same way as for covalent compounds (sum of atomic masses), but it does not represent a single molecule (not a molecular mass).
  • NaCl example:
    • Calculation: MextNaCl=1×22.99+1×35.45=58.44 amuM_{ ext{NaCl}} = 1 \times 22.99 + 1 \times 35.45 = 58.44 \text{ amu}
  • Aluminum sulfate, Al$2$(SO$4$)$_3$:
    • Ionic formula indicates Al$^{3+}$ and SO$_4^{2-}$ in a 2:3 ratio.
    • For calculation, rewrite as Al$2$S$3$O$_{12}$.
    • Calculation: M<em>extAl</em>2extS<em>3extO</em>12=2×26.98+3×32.06+12×16.00=342.14 amuM<em>{ ext{Al}</em>2 ext{S}<em>3 ext{O}</em>{12}} = 2 \times 26.98 + 3 \times 32.06 + 12 \times 16.00 = 342.14 \text{ amu}

Key Takeaways

  • Covalent compounds: formula mass = molecular mass; sums of all atom masses in the molecule.
  • Ionic compounds: formula mass = sum of atom masses in the formula; reflects bulk composition, not a single molecule.
  • Rewriting complex ionic formulas to a simpler unit can simplify calculation (e.g., Al$2$S$3$O${12}$ for Al$2$(SO$4$)$3$).