  Proportion or ratio of each element in a chemical compound  Percentage by mass of an element in a particular substance  Expressed as grams of element per 100 grams of chemical compound Percent composition or Percent by Mass

  Find formula mass  Find % of each element in the compound, must all add up to 100%  Part Whole Ex. 1 % Composition of MgCO 3

 1. Al 2 O 3 Find the percentages of Al and oxygen in this compound. 2. Ammonium phosphate. What is the percentage of hydrogen in this compound? What is the amount (in grams) of hydrogen? Percent Composition Practice

 1)Empirical  Simplest formula for a molecular compound  Smallest whole number ratio of elements in a compound  Many compounds have the same formula  Ex. CH 2 O 2)Molecular  Unique to a specific chemical compound  Displays the actual # of atoms of each element in a chemical compound  “true formula” 2 types of chemical formulas

 If we are given just the % composition of each element in a compound—how do we get the molecular formula ????

 1)Find the empirical formula 1)Use BOTH the empirical formula AND molecular mass to find the molecular formula. Pathway to the Molecular Formula

 1)Assume a 100g sample of chemical compound. Convert % of each element into grams. 2)Convert grams moles for each element 3)Divide each mole amount by the smallest amount. 4)Multiply each number by smallest integer to get whole numbers. Finding the empirical formula----

 Ex. 1 Adipic acid has 49.32% C, 43.84% O, and 6.85% H. Find the empirical formula.

  Find the molecular formula.  Need the empirical formula AND molecular mass.  Adipic molecular mass = 146 g/mol Ex. 1 The Sequel

 1. A compound has an empirical formula of C 2 OH 4 and a molar mass of 88 g/mol. Find the molecular formula. 2. Rubbing alcohol contains 60.0%C, 13.4% H, and the remaining mass is from oxygen. What is the empirical formula for rubbing alcohol? 3. Caffeine is found to contain 49.48% C, 5.19% H, 28.85% N, and 16.48% O by mass. The molecular mass of caffeine is g/mol. Find the empirical and molecular formula of this compound. Empirical Formula Practice

  Read pp , p. 117 #21, 31, 32, 39, 40, 41 Homework