Empirical Formulas Unit 5 Percent Composition n Percent

Empirical Formulas Unit 5

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

Sample Problem n An 18. 02 g sample of H 2 O contains 2. 02 g of H and 16. 00 g of O. What is the percent by mass for each element? n 11. 2 % H n 88. 80 % O

Calculating Percent Composition n Step #1: Calculate the molar mass of the compound n Step #2: Use the atomic mass of each element and the formula for percent composition to calculate the percent composition of each element n Example: Na. HCO 3

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

Calculating Empirical Formula n Step #1: Convert grams of each element to moles n Step #2: Divide each of these numbers by the smallest number n Example: n What is the empirical formula for a compound that contains 6. 93 g of oxygen and 0. 43 g of hydrogen?

Sample Problem n A sample of unknown origin contains 48. 64% C, 8. 16% H, and 43. 20 % oxygen. What is the empirical formula for this compound? n C 3 H 6 O 2

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

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

Sample Problem n The empirical formula for succinic acid is C 2 H 3 O 2. The molar mass of succinic acid is 118. 1 g/mol. What is the molecular formula for succinic acid? n C 4 H 6 O 4