1 CHEMICAL REACTIONS Example Hydrogen Oxygen Water H2 O2 H2O Note there is not enough hydrogen to react with oxygen It is necessary to balance equation reactants products 2 H2 O2 2 H2O balanced equation CONSERVATION OF MASS During a chemical reaction matter is neither created nor destroyed i e the number of atoms of each element remains constant BALANCING CHEMICAL EQUATIONS ensures of reactant atoms of product atoms C2H4 O2 CO2 Cu S8 Cu2S Fe2O3 C Fe Sc2O3 H2O Sc OH 3 CH3NH2 O2 H2O CO2 CO2 H2O N2 Sometimes it is more convenient to balance groups of atoms polyatomic ions than individual atoms AgNO3 CaCl2 Ca NO3 2 AgCl Balance NO3 ions rather than N and O atoms Ba ClO4 2 Na2SO4 Balance ClO4 and SO42 ions BaSO4 NaClO4 2 FORMULA AND MOLECULAR MASS Formula Mass Weight sum of atomic masses in chemical formula Molecular Mass Weight same as formula mass for molecular compounds technically not defined for ionic compounds Clarification of definitions often the words mass and weight are used interchangeably even though they are technically not the same thing often formula mass and molecular mass are used interchangeably even though they are technically not the same things Calculating Formula Mass Example C3H6 cyclopropane 3 12 011 amu 6 1 00794 amu 42 806 amu Example Ca OH 2 calcium hydroxide 40 08 amu 2 15 9994 amu 2 1 00794 amu 74 09 amu THE MOLE A mole is 6 022 x 1023 items 1 mole 6 022 x 1023 items Analogy 1 dozen 12 items 18 eggs 1 5 dz to convert from eggs from dozen we need to multiply by conversion factor Example How many eggs in 3 2 dozen Analogy 2 1 gross 144 items Example How many gross is 68 pencils 3 1 mole 6 022 x 1023 items 6 022 x 1023 is called Avogadro s number and is abbreviated NA 6 022 x 1023 molecules 1 molemolecule 12 044 x 1023 molecules 2 molemolecule Example How many moles of atoms is 7 43 x 1021 atoms Example How many moles of ions is 2 5 x 1025 ions Example How many molecules are in 8 333 mol of molecules MOLAR MASS Definition of Molar Mass Mass of 1 mole 12C is 12 000000 g BY DEFINITION Recall 1 atom of 12C is 12 000000 amu This is not a coincidence Definition of amu and a mole are made to ensure coincidence 1 amu 1 66 x 10 27 kg 1 NA grams 1 molC 12 12 000000 g 2 molC 12 24 000000 g 47 36 mol C 12 47 36 mol C 12 12 000000 g C 12 568 3 g C 12 mol C 12 ATOMIC MOLAR MASS 12 C 1 atom 12 amu 1 mol 12 g 24 Mg 1 atom 24 amu 1 mol 24 g 56 Fe 1 atom 56 amu 1 mol 56 g M 12C 12 g mol M 24Mg 24 g mol M 56Fe 56 g mol 4 AVERAGE ATOMIC MOLAR MASS Elements often have two or more naturally occurring isotopes Average atomic molar mass is average all atomic molar masses of the naturally occurring isotopes according to each isotope s relative abundance Except for units identical to average atomic mass Note mass of isotopes not integers because of nuclear forces Example Neon nuclide abundance 20 Ne 90 48 21 Ne 0 27 22 Ne 9 25 mass g mol 19 992 20 994 21 991 M Ne 0 9048 19 992 g mol 0 0027 20 994 g mol 0 0925 21 991 g mol 20 18 g mol Note example is the same as example for average atomic mass Example How many moles of atoms are in 96 3 grams of carbon Example How much mass does 0 0840 moles of uranium have FORMULA MOLAR MASS add molar masses for all atoms within a chemical formula Example ethene C2H4 M C2H4 2 x 12 011 g mol 4 x 1 00794 g mol 28 054 g mol Ethene ethylene is used to ripen fresh fruit It is also used to make polyethylene which is used to make milk jugs Example How many moles are in 538 g of Ba NO3 2 First calculate formula weight 2x 6x 137 33 g mol 14 0067 g mol 15 9994 g mol 261 35 g mol 538 g 538 g Barium nitrate is used to color fireworks green 1 mol 2 06 mol 26135 g 5 SCHEME Converting mass to moles to number M Molar mass Mass g NA Avogadro s number Moles mol Number atoms or molecules Note Molar mass and Avogadro s number are conversion factors Example How many atoms are in 20 21 g of He Example How many F atoms are in 13 11 g of CaF2 EMPIRICAL FORMULA FROM MASS PERCENT ANALYSIS Given Percent Mass Composition Find Empirical Formula Strategy 1 Assume 100 g of matter 2 Multiply 100 g by mass percent to find amount of each element 3 Convert mass of each element to moles using molar mass 4 Find whole number ratios by dividing each number of moles by lowest number of moles 6 Example Find the empirical formula for a compound with the following mass percentages 79 73 Cl 18 01 C 2 26 H 2 3 Cl 100 0 g x 0 7973 79 73 g C 100 0 g x 0 1801 18 01 g H 100 0 g x 0 0226 2 26 g H C Cl 4 Empirical Formula is C2H3Cl3 7 COMBUSTION ANALYSIS A mass of hydrocarbon is burnt with oxygen to produce CO2 H2O and N2 From the mass of CO2 H2O and N2 produced and mass of oxygen used find the empirical formula for the compound Ultimately we want to find molar ratios that we ll use to find the empirical formula 1 Convert mass of CO2 to moles of carbon atoms calculation yields moles of carbon in hydrocarbon sample CO2 g KOH s KHCO3 s 2 Convert mass of H2O to moles of hydrogen atoms because there are two moles of hydrogen to one mole of water multiply result by 2 to yield moles of hydrogen in hydrocarbon sample H2O g CaCl2 s CaCl2 2H2O s 3 Convert mass of N2 to moles of nitrogen atoms because there are two moles of nitrogen atoms in one mole of nitrogen molecules multiply result by 2 to yield moles of nitrogen in hydrocarbon sample 4 Calculate moles of oxygen in products moles of oxygen 2 moles of CO2 moles of H2O 5 Calculate moles of oxygen in hydrocarbon moles of hydrocarbon oxygen moles of oxygen atoms in products moles of oxygen atoms used 6 Divide molar amounts by smallest of the values to find molar ratios in terms of whole numbers 8 Example Find the empirical formula for a sample of hydrocarbon that produces 7 61 g of CO2 4 15 g of H2O and uses 7 392 g of O2 during combustion Moles of carbon 7 61g 1molCO2 44 0 g CO2 1molC 0 173molC 1molCO2 Moles of hydrogen 4 15g 1molH2O 18 0 g H 2O …