Determining Percent Composition Early chemists pulled apart the chemical elements in compounds and found they had different amounts of them. (Think Dalton!) They were able to determine the percent composition of elements in various compounds from the mass of the sample and the masses of each element found in the compound o Think of water and hydrogen peroxide – are the percent compositions of hydrogen and oxygen the same in each? Knowing the percent compositions of elements in a compound allows us to determine the ratio of elements in it

Determining Empirical Formula from % Composition Knowing the percent compositions of elements in a given compound then allows us to determine the ratio of elements in it by using the GFM=mole calcuation Example: Calculate the empirical formula of a substance that contains % sulfur and % oxygen by mass o When percentages are given, assume that they represent the number of grams in a 100 gram sample o In the above problem, o % sulfur means grams of sulfur o % oxygen means grams of oxygen

Grams cannot be compared directly, so the masses must be converted to moles o 1 mole of sulfur = 32.0 g/mol o 1 mole of oxygen = 16.0 g/mol Calculate the number of moles in the sample 50.0 grams S = 1.56 moles S 32.0 g/mole S 50.0 grams O = moles O 16.0 g/mole S

Determine the ratio (that’s what an empirical formula is) by dividing both by the smaller value 1.56 moles S = 1 S 1.56 moles moles O = 2 O 1.56 moles The empirical formula = 1 S and 2 O or SO 2

Practice 1 - Calculate the empirical formula of a substance that contains 80.0 % carbon and 20.0 % hydrogen by mass o How many grams are assumed to be in my sample?g o How many grams of carbon are there if there is 80% carbon by mass?g o How many grams of hydrogen are there if there is 20% hydrogen by mass?g Using the Periodic Table o What is the GFM of carbon?g o What is the GFM of hydrogen?g

Practice 1 - Calculate the empirical formula of a substance that contains 80.0 % carbon and 20.0 % hydrogen by mass Use the mole conversion formula: o How many moles of carbon are present? o How many moles of hydrogen are present? o What is the ratio of carbon to hydrogen? o What is the empirical formula?

Practice 2 You are working as a geochemist for a mining firm, and you have been handed a gram sample of the mineral pictured at the right. It could be chalcocite, which has two copper atoms to every one sulfur atom. Your sample is 20% sulfur and 80% copper. Determine the empirical formula and confirm or refute that this is chalcocite.

Practice 2 Empirical Formula of Chalcocite