Section 9.3—Analysis of a Chemical Formula How can we determine a chemical formula?

Percent Composition How is percent composition found?

What is Percent Composition?

Example #1 Example: An 8.5 g sample is composed of carbon and hydrogen. If 5.5 g of the sample is found to be carbon, what is the percent composition by mass of the carbon and hydrogen in the sample?

Example #1 If total = 8.5 g and C = 5.5 g - then H = 3.0 g Example: A 8.5 g sample is composed of carbon and hydrogen. If 5.5 g of the sample is found to be carbon, what is the percent composition by mass of the sample? If total = 8.5 g and C = 5.5 g - then H = 3.0 g

Let’s Practice #1 A sample is 57.0% by mass chlorine, how many grams of chlorine are present in a 27.5 g sample?

Let’s Practice #1 A sample is 57.0% by mass chlorine, how many grams of chlorine are present in a 27.5 g sample? 27.5 g   27.5 g 100 100

Example #2 Percent composition can also be determined from a chemical formula Example: Find the percent composition, by mass, of CaCl2

Example #2 Percent composition can also be determined from a chemical formula Example: Find the percent composition, by mass, of CaCl2 Ca Cl 1 2 40.08 g/mole 35.45 g/mole  = 70.90 g/mole + 110.98 g/mole So for 1 mole: total = 110.98 g Ca = 40.08 g Cl = 70.90 g

Let’s Practice #2 Find the percent composition, by mass, of NaNO3

Find the percent composition, by mass, of NaNO3 Let’s Practice #2 Find the percent composition, by mass, of NaNO3 Na N 1 22.99 g/mole 14.01 g/mole  = + 85.00 g/mole O 3 16.00 g/mole 48.00 g/mole