CHAPTER 10 FORMULA AND PERCENT COMPOSITION

USING ANALYTICAL DATA Scientist synthesize new compounds for many uses. Once they make new product, they must check its identity One way to carry out a chemical analysis is by providing a percent composition.

DETERMINING % COMPOSITON

EXAMPLE OF % COMP Ex: Find the % composition of Ca, O, and H in Ca(OH) 2

WHAT IS A HYDRATE? A hydrate is a chemical that has water molecules chemically bonded to it.

HOW CAN WATER BE REMOVED FROM HYDRATES? Heat is used as a catalyst to chemically remove the water from the compound, leaving the anhydrous solid. When no more water vapor is leaving the solid upon heating, the process of converting from hydrate to anhydrate is complete. Mass results can be analyzed.

HYDRATES A color change can sometimes indicate the chemical removal of water from the compound.

DETERMINE THE % COMP OF WATER IN HYDRATE What is the % comp of water in Cobalt(II) chloride hexahydrate? CoCl 2 · 6H 2 O

DETERMINING EMPIRICAL FORMULAS Data for the percent composition allow you to calculate the simplest ratio among the atoms found in a compound. Empirical formula is the simplest ratio of atoms in a compound

PERCENT COMPOSITION TO EMPIRICAL FORMULA Step 1- Change % to grams, assuming a 100g sample Step 2- Convert grams to moles Step 3- Divide all mole values by the smallest mole value to try and get all values into simple whole numbers Step 4- If mole values are not whole numbers after step 3, multiply all mole values by the same smallest whole number to achieve this Step 5- Write the empirical formula of the compound

EXAMPLE PROBLEM A compound is found to have 79.8% carbon and 20.2% hydrogen by mass. What is the empirical formula of this compound?

EXAMPLE A compound is composed of 58.8% C, and 9.8% H, and 31.4% O. What is the empirical formula of this compound?

MORE PRACTICE PROBLEMS 60.0% C, 13.4% H, 26.6% O. Find the empirical formula. A dead battery is found to contain a compound of 69.9% Mn and 30.4% O. What is the empirical formula? Magnetic iron oxide is 72.4% iron and 27.6% oxygen. What is the empirical formula?

EMPIRICAL FORMULAS NOT MOLECULAR FORMULAS In determining empirical formulas, the simplest ratio of atoms was determined. This is not necessarily the exact number of atoms in the compound which is known as the molecular formula. Additional information is needed to determine molecular formulas.

MOLECULAR FORMULAS Molecular formulas are made of single molecules. For many compounds the molecular formula is a simple whole number ratio of the empirical formula. The molar mass of a molecular formula is equal to the molar mass of an empirical formula multiplied by “n” (a whole number)

PROCESS Divide the experimental molar mass by the molar mass of the empirical formula, you can figure out the value of n needed to scale the empirical formula up to give the molecular formula. n(empirical formula)= molecular formula

EXAMPLE PROBLEM The empirical formula of a compound is CH 4 N. The mass of the molecular formula is g/mol. What is the molecular formula of this compound?

MORE EXAMPLE PROBLEMS Molar mass of 78 g/mol and empirical formula of CH, what is the molecular formula? A brown gas has the empirical formula of NO 2. Its experimental molar mass is 46 g/mol. What is the molecular formula?

EXAMPLE The lichens present on the bark of trees contain a substance called succinic acid. The organic acid is also present in fungi, which is used to make dyes and perfumes. Percent composition is 40.68% C, 5.08% H, 54.24% O. Its molecular mass is g/mol. What is its empirical and molecular formulas?