4 1 Chapter 4 Part 1 Stoichiometry Calculating Chemical Quantities This section of the notes includes Section 2 9 Sections 3 7 3 10 and Sections 4 1 4 3 in the Tro textbook The Mole Definition of mole the SI unit for amount of substance the quantity of a substance that contains as many molecules or formula units as there are atoms in exactly 12 g of carbon 12 1 Mole is a word 1 dozen eggs a couple of tacos 1 gross of pencils 1 mol of particles 1 mol of atoms 1 mol of electrons 1 mol of molecules Example How many moles of atoms are present in 9 5 x 10 15 atoms Example How many molecules are present in 2 35 mol of molecules 2 Mole relates to 1 mol C 1 mol Na 1 mol H2O 1 mol NH4 3PO4 This is called the and can be used to convert Molar mass Other terms for molar mass 4 2 A very useful formula for molar mass Example How many moles of carbon are in 25 0 g of carbon Example How many iron atoms are in 675 g of iron Example How many moles of water are in 25 0 g of water Example How many water molecules are in 675 g of water 3 The mole concept provides ratios There are two places to get mol to mol ratios 1 magnesium nitrate 2 2 C2H6 g 7 O2 g 4 CO2 g 1 Mole to Mole Ratios from Chemical Formulas How many hydrogen atoms are in 59 3 g of water How many nitrate ions are in 3 99 g of aluminum nitrate 6 H2O l 4 3 How many ions are in 15 0 g of ammonium carbonate Balancing Chemical Equations In order to correctly calculate quantities in chemical reactions the balanced equation must be used Examples Balance the following equations Fe3O4 H2 Fe2O3 C NH3 O2 Fe H2O Fe NO CO2 H2O Ammonium phosphate reacts with magnesium nitrate to form magnesium phosphate and ammonium nitrate One important reaction to know Combustion When a hydrocarbon a compound containing only C and H or a compound containing only C H and O undergoes complete combustion in oxygen the products are CO2 and H2O Example Write the balanced equation for the complete combustion of ethane C2H6 in oxygen Example When the balanced equation for the complete combustion of ethanol CH3CH2OH is written using the smallest whole number coefficients what is the coefficient on oxygen a b c d e 1 3 5 6 7 4 4 Reaction Stoichiometry Stoichiometry involves using the coefficients in a balanced chemical equation to determine quantities in chemical reactions Solid aluminum reacts with hydrochloric acid to form an aqueous solution of aluminum chloride and hydrogen gas Balanced equation This means Thus we can derive to ratios Example What mass of hydrogen gas can be formed if 12 0 g of aluminum reacts with excess HCl Example What mass of oxygen gas is required for the complete combustion of 29 0 g of butane C4H10 Example What mass of oxygen gas is required for the complete combustion of 10 0 g of cyclopentanol C5H9OH Percent Yield The amount of product collected at the end of a reaction the actual yield is often less than the yield that we predict we should get using reaction stoichiometry the theoretical yield Percent yield 4 5 Example Magnesium carbonate decomposes on heating to form magnesium oxide and carbon dioxide Suppose that 50 0 g of magnesium carbonate was heated and that 20 0 g of magnesium oxide was subsequently collected What was the percent yield of this reaction Limiting Reactant Bologna Sandwich Example If you have 50 slices of bread and 50 slices of bologna how many sandwiches can you make If you make 17 sandwiches what is your percent yield Example In the balanced equation below how many grams of solid iron can be formed if 50 0 g of Fe2O3 reacts with 10 0 g of CO Fe2O3 s 3CO g 2Fe s 3CO2 g 4 6 Example In the balanced equation below suppose that 25 8 g of Fe2O3 reacts with 15 0 g of CO Which reactant is the excess reactant and how many grams of the excess reactant remain Fe2O3 s 3CO g 2Fe s 3CO2 g Percent Composition of Compounds Recall our Junior High definition of percent When we refer to percent composition in chemistry we usually mean percent What is the percent hydrogen in water a 11 2 b 33 3 c 50 0 d 66 7 What is the percent nitrogen in ammonium phosphate Empirical and Molecular Formulas A molecular formula shows An empirical formula shows Example The molecular formula of benzene is C6H6 What is the empirical formula of benzene Example Suppose that a compound with the empirical formula CH2O has a molar mass of 120 g mol What is the molecular formula of this compound 4 7 Two keys to solving empirical formula problems 1 2 Example What is the empirical formula of a compound which consists of 52 13 C 13 14 H and 34 73 O by mass If you are given the percent composition and the molar mass the problem is easier Instead of starting with 100 g of unknown start with Example A certain compound consists of 38 70 C 9 74 H and 51 56 O by mass The molar mass of this compound is 62 07 g mol What is the molecular formula of this compound Combustion Analysis In combustion analysis all of the carbon in the unknown is converted to and all of the hydrogen in the unknown is converted to Example An unknown compound consists of carbon hydrogen and oxygen When a 1 125 g sample of the unknown was analyzed by combustion analysis 1 649 g of CO2 and 0 675 g of H2O were formed If the molar mass of this unknown is 180 2 g mol what is the molecular formula