a calculation used to determine the contribution of each element as a %age of the total mass of the compound There are 2 ways we can do this: 1) You are given the mass of the compound and each of the masses of the elements the compound is made up of 2) You are given the chemical formula of the compound

1) Percentage Composition using masses of elements and compounds -is similar to how you calculate your mark on a test -to calculate the percentage composition of each element in a compound, you divide the mass of each element by the mass of the compound it is in Ex.1 - What is the percentage composition by mass of sodium chloride if 3.45 g of sodium metal react with 5.33 g of chlorine gas to produce 8.78 g of sodium chloride.

2) Steps to find the percentage composition from the chemical formula of a compound 1. Calculate the mass of each element in the compound 2. Determine the mass of the entire compound 3. Divide the mass of each element by the total mass of the compound to get a %age composition for each element

Ex.2 - What is the percentage composition of nitric acid? 1. Calculate the mass of each element in the compound 2. Determine the mass of the entire compound 3. Divide the mass of each element by the total mass of the compound to get a %age composition for each element

the simplest formula of a substance represents the ratio in which the atoms are present in a compound does not indicate the actual # of each type of atom within a compound it is like the most simplified version of a chemical formula For ex. Ethyne (C 2 H 2 ) and Benzene (C 6 H 6 ) have the same empirical formula It is:

Ex.1 - What is the empirical formula of a compound that is found to contain 92.3% carbon and 7.7% hydrogen

Ex.2 - What is the empirical formula of a compound that is found to contain 2.2% H, 26.7%C and 71.1% O?

indicates the # of atoms of each element found in the molecule can only be written for compounds that are molecule in some cases the empirical and molecular formula may be the same ex. CO 2, H 2 S How are we going to do this -Divide the molecular mass of the compound given by the molecular mass of the empirical formula -Then multiply the empirical formula by this #

Ex. CH 2 O is the empirical formula of a certain compound whose molecular mass is 180 u. What is the molecular formula of the compound?

Ex.2 The analysis of a compound shows that is made up of 21.9% Na, 45.7% C, 1.9% H and 30.5%. What is the molecular formula of the compound if its molecular mass is 210u?

Homework P g.184 # 5,6,8 (practice ?s) 184 #3,4 (section ?s) Pg.186 # 2,5 189 #3-5 Pg.193 #8-10 Have a great weekend !