Representation of Compounds
High-Yield Summary
- Chemical formula shows atom types/ratios (e.g., H₂O); structural formula shows spatial arrangement/bonding (H–O–H).
- Law of constant composition: every pure sample of a compound has the same elements in the same mass proportions, regardless of source.
- Empirical formula = simplest whole-number ratio of elements; molecular formula = actual atom count per molecule. They can match (H₂O) or differ by a whole-number multiplier (glucose: CH₂O → C₆H₁₂O₆).
- Ionic compounds (NaCl, CaCO₃) are represented ONLY by an empirical formula — no discrete molecule exists.
- % Composition = (mass of element ÷ molar mass of compound) × 100%; combined with molar mass, it can be reversed to find the molecular formula.
Percent Composition → Molecular Formula
- 1Convert each element's mass % to moles (treat as grams per 100 g of compound, divide by atomic mass).
- 2Reduce the mole ratio to the simplest whole-number ratio → this is the empirical formula.
- 3Divide the compound's actual molar mass by the empirical formula's mass to find the whole-number multiplier, then scale up.
Percent Composition
% Composition = (Mass of element in formula / Molar mass of compound) × 100%
- Mass of element = Total mass contributed by that element in the formula
- Molar mass = Molar mass of the whole compound
- Worked example: 40.0% C, 6.7% H, 53.3% O, molar mass ≈ 180.16 g/mol → mole ratio ≈ 3.33:6.65:3.33 → 1:2:1 → empirical CH₂O (≈30.03 g/mol) → 180.16/30.03 ≈ 6 → molecular formula C₆H₁₂O₆ (glucose).
Empirical vs. Molecular Formula — Worked Cases
| Compound | Empirical / Molecular |
|---|---|
| Water | H₂O / H₂O — identical |
| Hydrogen peroxide | HO / H₂O₂ — molecular formula is 2× empirical |
| Glucose | CH₂O / C₆H₁₂O₆ — molecular formula is 6× empirical |
Common MCAT Trap
- Percent composition gives the SAME answer whether calculated from the empirical or molecular formula — the element ratio doesn't change.
- Don't try to write a 'molecular formula' for an ionic compound like NaCl or CaCO₃ — only the empirical formula applies, since no discrete molecule exists.
- The empirical-to-molecular multiplier must come out to a whole number (or very close to it, accounting for rounding) — if it doesn't, recheck the mole ratio calculation.
Quick Recall
What does the law of constant composition guarantee about any pure sample of a compound?
Hydrogen peroxide's molecular formula is H₂O₂ — what's its empirical formula, and why do they differ?
Why can't calcium carbonate (CaCO₃) have a 'molecular formula' distinct from its empirical formula?