Empirical and molecular formulas C-notes on page 62

Finding Empirical Formulas © 2009, Prentice-Hall, Inc.

Calculating Empirical Formulas One can calculate the empirical formula from the percent composition. © 2009, Prentice-Hall, Inc.

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. © 2009, Prentice-Hall, Inc.

Calculating Empirical Formulas Step #1: Always assume you have 100.00 g of the substance, in this case, para-aminobenzoic acid, divide each mass by the molar mass of the element. 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 14.01 g 1.01 g 16.00 g © 2009, Prentice-Hall, Inc.

Calculating Empirical Formulas Step #2: Calculate the mole ratio by dividing by the smallest number of moles: C: = 7.005  7 H: = 6.984  7 N: = 1.000 O: = 2.001  2 5.105 mol 0.7288 mol 5.09 mol 1.458 mol © 2009, Prentice-Hall, Inc.

Calculating Empirical Formulas These are the subscripts for the empirical formula: C7H7NO2 © 2009, Prentice-Hall, Inc.

Molecular formula Once you have calculated the empirical formula, find the molar mass of the empirical formula. To find the molecular formula, you must be given the molecular weight of the compound. Use the following formula: whole number ratio= molecular weight empirical formula weight * Multiply whole number ratio by each part of your empirical formula

Example: These are the subscripts for the empirical formula: C7H7NO2 The molar mass of PABA is 137.12 g/mol Find the molecular formula Find the molecular weight of the empirical formula (138.09) Use the formula: the whole number ratio= 1 Multiply the empirical formula by ratio Molecular formula is C7H7NO2 In this example they are the same but not always!!!!!