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Calculating Empirical and Molecular Formulas

This lesson focuses on calculating empirical and molecular formulas for different compounds using principles of moles, mass, and ratios. Students will learn how to convert mass composition to moles, determine empirical formulas from percent composition, and calculate molecular formulas using empirical formulas and molar mass.

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Calculating Empirical and Molecular Formulas

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  1. Georgia Performance Standards: SC2 (c, d & e): Students will relate how the Law of Conservation of Matter is used to determine chemical composition in compounds and chemical reactions. c. Apply concepts of the mole and Avogadro’s number to conceptualize and calculate the empirical & molecular formulas, Mass, moles and molecules relationships, and molar volumes of gases. Essential Questions: How do you determine the empirical and molecular formulas for various compounds? - Performance Tasks Relate the concept of atomic mass units to empirical & molecular formulas for various compounds. Calculate the empirical & molecular Formulas for various compounds. Chapter 9 Chemical Quantities: Empirical & Molecular Formulas

  2. Calculating Empirical Formulas One can calculate the empirical formula from the percent composition. - Empirical formula: the symbols for the elements combined in a compound, with subscripts showing the smallest whole-number mole ratio of the different atoms in the compound

  3. Empirical Formulas of Ionic and Molecular Compounds: • In ionic compounds the formula unit is usually the compound’s empirical formula. • In molecular compounds: empirical formula does not always indicate the actual numbers of atoms present in each molecule • In this case the number of atoms given by the molecular formula corresponds to the empirical ration multiplied by two.

  4. From Mass Composition to Empirical Formulas: • Convert mass composition of each element to a composition in moles by dividing by the appropriate molar mass. • Calculate the smallest whole number ratio of atoms by dividing each number of moles by the smallest number in the ration (round to the nearest whole number). EX: A 10.150g sample of a compound contains 4.433g of P. How many grams of oxygen does it contain? What is the empirical formula?

  5. From % Composition to empirical Formulas: • Convert % composition to mass composition (Assume 100g of sample of compound) • Calculate the amount of each element in the sample • Convert mass composition of each element to a composition in moles by dividing by the appropriate molar mass. • Calculate the smallest whole number ration of atoms by dividing each number of moles by the smallest number in the ration (round to nearest whole number) EX: A compound contains 32.38% Na, 22.65% S, and 44.99% O. Find the empirical formula of this compounds.

  6. Calculating Empirical Formulas The compound para-aminobenzoic acid (you may have seen it listed as PABA on your bottle of sunscreen) is composed of carbon (61.31%), hydrogen (5.14%), nitrogen (10.21%), and oxygen (23.33%). Find the empirical formula of PABA.

  7. Assuming 100.00 g of para-aminobenzoic acid, C: 61.31 g x = 5.105 mol C H: 5.14 g x = 5.09 mol H N: 10.21 g x = 0.7288 mol N O: 23.33 g x = 1.456 mol O 1 mol 12.01 g 1 mol 14.01 g 1 mol 16.00 g 1 mol 1.01 g Calculating Empirical Formulas

  8. C: = 7.005  7 H: = 6.984  7 N: = 1.000 O: = 2.001  2 5.09 mol 0.7288 mol 0.7288 mol 0.7288 mol 5.105 mol 0.7288 mol 1.458 mol 0.7288 mol Calculating Empirical Formulas Calculate the mole ratio by dividing by the smallest number of moles:

  9. Calculating Empirical Formulas These are the subscripts for the empirical formula: C7H7NO2

  10. Calculating Molecular Formulas: • Molecular formula = the actual formula of a molecular compound • An empirical formula may or may not by a correct molecular formula • EX: BH3 = empirical formula B4H12= molecular formula

  11. Writing the molecular formula from the empirical formula: • Determine the compound’s formula mass. • Divide the formula mass the the empirical formula mass to get “X”. • Multiply the value for “X” by the empirical formula X (empirical formula) = molecular formula “X”= a whole number multiple indicating the factor by which the subscripts in the empirical formula must be multiplied to obtain the molecular formula X (empirical formula mass) = molecular formula mass Molecular Formula Mass = Molar Mass (remember!!!)

  12. EX: Calculating Molecular Formulas: • The empirical formula of a compound of phosphorus and oxygen was found to be P2O5. The molar mass of the compound is 283.89 g/mol. What is the compound’s molecular formula? • Determine the molecular formula of the compound with an empirical formula of CH and a formula mass of 78.110 amu. • A sample of a compound with a formula mass of 34.00 amu’s is found to consist of 0.44 g of H and 6.92 g of O. find its molecular formula.

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