Empirical Formulas Unit 5

Percent Composition Percent composition is a number that tells us how much of an element makes up a compound The formula used to calculate percent composition is:

Sample Problem An 18.02 g sample of H2O contains 2.02 g of H and 16.00 g of O. What is the percent by mass for each element? 11.2 % H 88.80 % O

Calculating Percent Composition Step #1: Calculate the molar mass of the compound Step #2: Use the atomic mass of each element and the formula for percent composition to calculate the percent composition of each element Example: NaHCO3

Empirical Formula If we know the percent composition for each element of a compound, we can calculate the compound's empirical formula An empirical formula is the formula with the smallest whole number ratio of the elements

Calculating Empirical Formula Step #1: Convert grams of each element to moles Step #2: Divide each of these numbers by the smallest number Example: What is the empirical formula for a compound that contains 6.93 g of oxygen and 0.43 g of hydrogen?

Sample Problem A sample of unknown origin contains 48.64% C, 8.16% H, and 43.20 % oxygen. What is the empirical formula for this compound? C3H6O2

Molecular Formula A molecular formula tells you exactly the number of atoms present in a molecule of a compound Molecular formulas can be calculated by using empirical formulas

Calculating Molecular Formulas Molecular formulas can be calculated using the equation below:

Sample Problem The empirical formula for succinic acid is C2H3O2. The molar mass of succinic acid is 118.1 g/mol. What is the molecular formula for succinic acid? C4H6O4