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Empirical and Molecular Formulas

Sep 6, 2025

Overview

This lesson explains how to calculate empirical and molecular formulas from elemental composition data, using step-by-step examples common on tests.

Empirical Formula Calculation

  • Composition data is given as mass (grams) or mass percentage; percentages are assumed from a 100g sample.
  • Convert each element’s mass to moles using: moles = mass / molar mass (from the periodic table).
  • Find the mole ratio by dividing each mole value by the smallest mole value among the elements.
  • Round all ratios to the nearest whole number to get subscripts for the empirical formula.
  • Write the empirical formula by placing these subscripts after each element symbol.

Example 1

  • Sample: Na (36.5g), S (25.4g), O (38.1g).
  • Moles: Na = 1.59, S = 0.79, O = 2.38.
  • Ratios after division by smallest (0.79): Na = 2, S = 1, O = 3.
  • Empirical formula: Na₂SO₃.

Molecular Formula Calculation

  • Find the molar mass of the empirical formula by summing atomic masses × subscripts.
  • Divide the given (actual) molar mass by the empirical molar mass; answer should be a whole number.
  • Multiply all subscripts in the empirical formula by this factor for the molecular formula.

Example 1 (continued)

  • Empirical formula mass = 126g.
  • Molecular mass given = 378g; 378 ÷ 126 = 3.
  • Molecular formula: Na₆S₃O₉.

Example 2

  • Sample: C (49.5g), N (28.8g), H (5.2g), O (16.6g).
  • Moles: C = 4.125, N = 2.06, H = 5.2, O = 1.0375.
  • Ratios: C = 4, N = 2, H = 5, O = 1.
  • Empirical formula: C₄N₂H₅O.
  • Empirical mass = 97g; given molar mass = 194g; 194 ÷ 97 = 2.
  • Molecular formula: C₈N₄H₁₀O₂.

Example 3

  • Sample: C (19.95g), H (3.35g), O (26.7g).
  • Moles: C = 1.66, H = 3.35, O = 1.67 (all divided by 1.66).
  • Ratios: C = 1, H = 2, O = 1.
  • Empirical formula: CH₂O.
  • Empirical mass = 30g; given molar mass = 60g; 60 ÷ 30 = 2.
  • Molecular formula: C₂H₄O₂.

Key Terms & Definitions

  • Empirical Formula — The simplest ratio of elements in a compound.
  • Molecular Formula — The actual number of atoms of each element in a molecule.
  • Moles — Quantity of substance calculated as mass divided by molar mass.
  • Molar Mass — Mass in grams of one mole of a substance.

Action Items / Next Steps

  • Practice calculating empirical and molecular formulas with given exercises.
  • Review periodic table to quickly find atomic masses for calculations.

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