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Molar Mass, Percent Composition, Empirical & Molecular Formulas

Learn about calculating molar mass, percent composition, and how to determine empirical and molecular formulas. Understand the different types of chemical formulas and how to write structural formulas using Lewis dot diagrams.

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Molar Mass, Percent Composition, Empirical & Molecular Formulas

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  1. Unit 6.5 • Molar Mass • Percent Composition • Empirical Formulas • Molecular Formulas

  2. Molar Mass • Add rounded atomic weights. Multiply them, if needed, by the subscript. Ex) MgBr2

  3. Percent Composition • Found on Table T % composition = mass of part x 100 mass of whole Mass of whole = solute(solid/gas) + solvent (liquid)

  4. Chemical Formulas There are 3 types of chemical formulas: • Structural • Empirical • Molecular

  5. 3 Types of Formulas • Molecular Formulas show the actual number of atoms needed to make the formula. • Empirical formulas are used to show the smallest number of atoms needed to make the molecule. The empirical formula denotes the smallest ratio possible.

  6. Structural Formulas and Lewis Dot Diagrams for Covalent Bonds • Shows which atoms are bonded together • Lewis dot diagrams are used for this and are called structural formulas Some structural formulas are shown here:

  7. Using Dot Diagrams to Write Structural Formulas Using Lewis dot diagrams: Step 1: Count the total number of valence electrons for the entire molecule Step 2: Place the electrons around the atoms so that they all have 8. You may have to place more than 1 pair between atoms. Step 3: Then replace the pairs with dashes between molecules as shown below: Note: Not all structural formulas show the unshared electron pairs

  8. Simplifying Empirical Formulas Some formulas will need to be simplified. Ex: Step 1: S+6 and O-2 Step 2: S2O6 divide both subscripts by highest common denominator (2) Step 3: SO3

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