1Chemistry: Reactions Reading: Supplemental Textbook Materials Chapter 9Pages 173-190, 196-201Volume Relationships• Gases react in small whole number quantities• Avogadro’s hypothesis: chemicals react in consistent, small whole number ratios• Equal volumes of gases at the same temperature and pressure have the same number of moleculesAvogadro’s hypothesis Combustion of propaneC3H8+ 5 O2Æ 3 CO2+ 4 H2O• How much (what volume of) oxygen is needed to burn 0.556 L of propane?• Ratio of Oxygen molecules to propane molecules is 5:1• 0.556 L C3H8x 5 L O2=2.87 L O21 L C3H8Problem 4 in class• Combustion of propaneC3H8+ 5 O2Æ 3 CO2+ 4 H2OCalculate how much CO2 is produced when 2 L of propane is burnedAtomic vs. Molecular Weight• Atomic weight is averagein u, atomic units• Atomic mass—specified by isotope• Molecular mass is the mass of one mole of molecules in grams• 6.0221367x1023molecules -- Avogadro’s #2Molar mass of substances• Mass of one mole of molecules of substance• Numerically identical to molecular mass of molecules in atomic mass unitsCalculate Molar Mass• O atomic weight 15.9996• O-16 atomic mass 16 u• Molecular oxygen-16 O2 atomic mass 32 u• Molecular O2molar mass 32 g/moleCalculate Molar Mass• CO2molecular mass• C = 12 g/mole, •O2 = 32 g/mole•CO2= 12 + 32 = 44 g/moleMolar Mass of C, O2, CO2• 1 mole C = 12 g•1 mole O2= 32 g• 1 mole CO2= 44 g22COOC →+Example Problem• What is the molar mass of each substance in this equation?22NOONO →+What is the molar mass of each substance in this equation?• 14 + 16 = 30 g N2• 16x2 = 32 g O2• 14 + 32 = 46 g NO222NOONO →+3Problem 5 in class• Calculate the molar mass of propane C3H8• assume molar mass of C = 12 g/mole• assume molar mass of H = 1 g/moleMoles calculated from Grams• Find the molar mass of substanceNa=23 g/mole• Multiply molar mass times # of moles• 23 g/mole x 0.250 moles = 5.75 gCalculate Moles from GramsNumber of moles of 176 g of CO2• Molar mass of CO2= 44 g/mole• If you multiply, you get a mess– 176 g x 44 g/mole results in units of g2/mole– UNITS Alert you that you made an errormolesgmolegg2774444176 =⋅Moles calculated from Grams• Instead divide 176 g by 44 g/mole• Or look at it as:176 g x (1 mole44 g)= 4 moles CO2molegg44176Problem 6 in classHow many moles of Si-28 are there in 140 g of Si-28?Be SURE to include UNITS!!Moles of substances4Standard temperature and pressure•0oC• Sea level pressure (29.92 “ Hg)• 1 mole of gas• 22.4 Liters about 0.8 ft3• Has mass of its formula massMass to moles relationshipMass of materials in chemical equationsDetermine molar ratios from balanced equationCalculate molar mass of reactants and productsConvert mass of quantity given to molesConvert moles of unknown from molar mass ratiosConvert back to grams from this molar amountProblemMolar ratio 1:1:1 4 grams oxygen– Grams carbon consumed?– Grams carbon dioxide produced?22COOC →+ProblemMolar ratio 1:1:1 4 grams oxygen•1 mole O2= 32 g22COOC →+22 125.032 1 4 OmolesgmoleOg =⋅ProblemMolar ratio 1:1:1 0.125 moles O2in 1:1 ratio0.125 moles C0.125 moles CO222COOC →+5Example Problem0.125 moles CGrams carbon consumed?CgmolegCmoles 5.1 112 125.0 =⋅22COOC →+22 5.5 144 mole 125.0 COgmolegCO =⋅Problem• 30 grams NO• How many grams NO2produced?22NOONO →+Balance EquationDetermine molar ratios 2:1:2Find molar mass of each component N=14 g, O=16 g, NO=30 g, NO2=46 gProblem• Find moles of quantity given30 g NO = I mole• Find moles of unknown• Molar ratios 2:2 reduces to 1:11 mole NO2How many grams?1 mole NO22222 NOONO →+2 g 46mole 1 46NOg=⋅ProblemOHSOOSH2222+→+• 32 grams SO2• How many grams O2used?Problem• Balance first• Then determine molar ratios•2 SO2 to 3 O2OHSOOSH2222+→+OHSOOSH22222232 +→+Problem• Find molar masses– 2x1(H)=2 +32(S)= 34 g/mol H2S – 2x16(O)= 32 g/mol O2– 32(S)+32(O2) = 64 g/mol SO2– 2x1(H) + 16 (O) = 18 g/mol H2OOHSOOSH22222232 +→+6ProblemOHSOOSH22222232 +→+• 32 g SO2needs how many grams O2?• How many moles is 32 g SO2?22SO 5.0g 64 1SO g 32 molesmole=⋅ProblemOHSOOSH22222232 +→+• 32 g SO2needs how many grams O2?• How many moles O2 is needed?• 0.5 moles SO2in 2:3 ratio with O222SO 5.0g 64 1SO g 32 molesmole=⋅Proportions• Mathematical device to compare ratios• Be sure you keep same:same in columns and rowsExampleorgg=amountmolar amountmolar ggamountmolar amountmolar =Proportions• correctly organized• cross-multiply to solveSolution to NO22222 moles 0.5 moles moles 2 moles 3SOOxSOO=• correctly organized• cross-multiply to solve• 0.75 moles of O2react with 0.5 moles SO2Solution• 0.75 moles of O2is how many grams?22 g 24mole1g 32 moles 75.0 OO
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