Molecules and Moles
High-Yield Summary
- A molecule is 2+ atoms held together by covalent bonds; a compound is 2+ DIFFERENT elements bonded (ionic or covalent).
- Ionic compounds form a 3D crystal lattice, not discrete molecules — so they use formula unit / formula weight instead of molecule / molecular weight.
- Equivalents = moles of a specific species of interest (H⁺ in acid-base, electrons in redox) produced or consumed per mole of compound.
- Gram equivalent weight = molar mass ÷ n, where n = equivalents per mole of compound.
- Normality (N) = equivalents/liter = molarity (M) × n — most often applied to H⁺ concentration on the MCAT.
Molecule vs. Compound — Overlap Cases
| Substance | Molecule? / Compound? |
|---|---|
| CO₂ | Both — a molecular compound |
| O₂, N₂ (diatomic elements) | Molecule, but NOT a compound (only one element) |
| NaCl (ionic) | Compound, but NOT a molecule (forms a crystal lattice, no discrete unit) |
Equivalent Weight and Normality
Gram Equivalent Weight = Molar Mass / n | Equivalents = Mass (g) / Gram Equivalent Weight | N = M × n
- n = Number of equivalents (of the species of interest) produced per mole of compound
- N = Normality (equivalents per liter)
- M = Molarity
- H₂SO₄: n = 2 (produces 2 mol H⁺ per mol), gram equivalent weight = 98/2 = 49 g/equivalent.
- 1 M H₂SO₄ = 2 N with respect to H⁺; 1 M HCl (n = 1) = 1 N with respect to H⁺.
Common MCAT Trap
- Don't call NaCl a 'molecule' — ionic compounds have no discrete, separable bonded unit; use 'formula unit' instead.
- n (equivalents per mole) isn't always 1 — it depends on the reaction context: H₂SO₄ gives n = 2 for H⁺, but the same molecule could have a different n in a redox context (electrons transferred).
- Formula weight and molecular weight are calculated identically (sum of atomic masses) — the terminology difference is about ionic vs. molecular substances, not the math.
Quick Recall
Why isn't NaCl considered a molecule?
What is the gram equivalent weight of H₂SO₄ (molar mass ≈ 98 g/mol)?
What is the normality of a 1 M HCl solution with respect to H⁺?