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Unit – The Mole

Learn about the mole concept, formula mass, percent composition, empirical and true molecular formulas, and stoichiometry in chemistry. Helpful hints and step-by-step guides included.

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Unit – The Mole

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  1. Unit – The Mole Formula Mass– The total mass of the formula for a compound. - To calculate formula mass, multiply the number of atoms of each element by the atomic mass (rounded to two decimal places) and then sum them all up.

  2. Percent Composition • Is useful to know what amounts (or percentages) of a compound are contributed by each element present in that compound. % comp. = mass of element (part) ------------------------------------ X 100 mass of compound (whole)

  3. The Mole - The mole is a unit of quantity (how much) of something is present. It has 3 definitions.

  4. 1 mole = atomic mass (element = periodic table) or formula mass (compound = calculated) in grams. • 1 mole = 6.02 X 1023atoms or molecules. (6.02 X 1023 = Avogadro’s number) • 1 mole = 22.4 liters iff the material is a gas and is at STP (standard temperature and pressure).

  5. Helpful Hints in Solving Mole Problems ***** If “GRAMS” appear in the problem and you have a “COMPOUND”, you must calculate the “FORMULA MASS”! ***** If moles do “NOT” appear in the problem you “NEED” a path.

  6. Empirical Formula • Empirical formula = simplest formula or smallest ratio of atoms. (ex) C6H6 EF = CH

  7. Steps to Determine the Empirical Formula • Convert all info to moles. • Divide answers to step # 1 by the smallest. • If the answers to step # 2 are close to whole numbers, round them off and use them as the subscripts for your formula. • If the answers to step #2 are not close to whole numbers, multiply by the smallest whole number that gives you a whole number. Then, use those as the subscripts to your formula.

  8. True (Molecular) Formulas • True molecular formulas are some ratio of an empirical formula. • EF = empirical formula • FM = formula mass (mass of the EF) • TMF = true molecular formula • MM = molecular mass (mass of the TMF)

  9. Steps to Determining True Molecular Formulas • Find the EF. • Calculate the FM • Divide the MM/FM = # • Multiply the subscripts of the EF by the number obtained in step #3 = TMF.

  10. Stoichiometry • Problems using balanced equations. • There are 3 types of Stoichiometry problems: • Mass – Mass • Mass – Volume • Volume - Volume

  11. Steps Used to Solve Stoichiometry Problems • Write the correct balanced equation. • Convert all given info to moles given. • Convert all moles given to moles sought. • Convert all moles sought to info sought. In other words, info given -- moles given -- moles sought -- info sought

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