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Chemistry Stoody Guide

Empirical & Molecular Formulas

  1. Convert between composition by mass and the empirical formula of a compound
  2. Determine the molecular formula, including determining the molar mass of a compound, from experimental data
  3. Explain the difference between empirical and molecular formulas”

Convert Between Composition By Mass and Empirical Formula

  1. Composition by mass: compute percentage composition based on ratios of AMU values of each element
  2. Empirical formula: find how many moles of each element there are; ratios convert to simplest formula (e.g. )
  3. If given percentages, assume weight to be any number (probably 100g) to find moles.

Determining Molecular Formula (Given Empirical Formula)

  1. Do guess and check until the molecular mass of the formula is the same (closee enough) to the given molecular mass.
  2. To find the molar mass, find the molecular mass and divide that over 1 mole.

The Difference Between Empirical and Molecular Formulas

  • Empirical is the smallest possible ratio, with a coefficient.
  • Molecular formula shows how many of each atom