Chapter 3: Chemical Formulas and Composition of CompoundsChapter 3: Chemical Formulas and Composition of CompoundsExam 2 Concepts You Should Know:Chemical FormulaUse of Chemical FormulasTypes of Chemical FormulasMolecular Weight (aka Formula Weight)Working with FormulasEmpirical FormulaPercent CompositionDetermination of Empirical FormulaDetermination of Molecular FormulaDetermination of Empirical Formula by Combustion AnalysisCombustion Analysis ExampleCombustion Analysis ExampleHow does this work?Practice ProblemStoichiometry StrategyWhat You Should Know What You Should Know What You Should KnowChapter 3: Chemical Formulas and Composition of CompoundsCH110 FA11 SAS 1Prepare for Recitation September 29thALEKS Objective 5 due September 27thANGEL Quiz 5, September 29thLecture 13: September 21stLecture 14: September 23rdLecture 15: September 26thRead: Ch. 2.6, 3.3-3.5, 9.1-9.3Additional Preparation: BLB 3: 25,29,35,37,51,53; Packet 3: 1-9BLB 9: 23,25,27,38; Packet 9: 1-7CH110 FA11 SAS 2By the end of Today’s Lecture you should know:Relationship between molecular and empirical formulaCombustion AnalysisFormula and molecular weightsStoichiometry problemsWhat does a structural formula tell us about the composition of a compound?What does the empirical formula tell us about the composition of compounds?Remember: structure affects function!How do chemists determine the composition of unknown compounds?KEY QUESTIONS: THE BIG PICTUREChapter 3: Chemical Formulas and Composition of CompoundsExam 2 Concepts You Should Know:CH110 FA11 SAS 3• How do we determine the composition of a compound from the various chemical formulas?• What is combustion analysis?Chapter 3: Chemical Formulas AgainChapter 25: Organic MoleculesChapter 9: Bonding and Geometry• What are the properties of hydrocarbons?• What do the functional groups we learned about look like and how do they affect properties?• What are the shapes of the electron domain and molecular geometries we have studied?• What is molecular polarity and how is it determined by shape?• What are sigma and pi bonds?• How do hybrid orbitals relate to molecular shape and bond angles?• What are the IMFs we have learned about?• How do IMFs impact boiling and melting points?Chapter 11: Intermolecular ForcesChemical FormulaCH110 FA11 SAS 4Chemical Formula: Subscripts denote the number of atoms per moleculeO2:O3:Use of Chemical FormulasCH110 FA11 SAS 5NomenclatureSubscripts Coefficients (the numbers that come before the formula)Conservation of Mass2H2+ O2 2H2OBalance your equations by changing the __________ NOT the _________• Matter is like energy: it is not created or destroyed in a reaction• Same mass for products and reactants• Same number of atoms for products and reactants“Stoichiometry” problems apply this rule.Types of Chemical FormulasCH110 FA11 SAS 6H2O H2O2O2CH3CH2OHMolecular Formula: Empirical Formula: Structural Formula:Molecular Weight (aka Formula Weight)CH110 FA11 SAS 7Formula Weight: MgCO3# of atomic Atom atoms weight weightMg 1 24.3 24.3C 1 12.0 12.0O 3 16.0 48.0Total 84.3 amuMgCO3formula weight = 84.3 amumolar mass = 84.3 g/molMolar Mass:Working with FormulasCH110 FA11 SAS 8• How many atoms are in the chemical formula Mg3(PO4)2?a) 3b) 5c) 10d) 13• How many moles of Mg3(PO4)2are in a 10.0 g sample?a) 0.038 molesb) 0.059 molesc) 0.043 moles• How many atoms are in 10.0 g of Mg3(PO4)2?a) 2.59 x 1022atomsb) 3.58 x 1022atomsc) 2.29 x 1022atomsd) 2.97 x 1023atomsEmpirical FormulaCH110 FA11 SAS 9How do we find the molecular formula from experimental data?• Relative ratio from experiment• Weight of whole molecule to compare w/ empirical formulaEmpirical Formula: a formula showing the relative abundance of atoms in a compound using the smallest possible whole number ratios.Percent CompositionCH110 FA11 SAS 10The connection between empirical formula and molecular formula!% of element = x100What is the mass percent of each element in H3PO4?(# atoms)(atomic weight)FW of compoundH = 1.008 amu P = 30.97 amu O = 16.00 amu%P = 30.97x 100 =%O = 4(16.00)x 100 =%H = 100 – =Determination of Empirical FormulaCH110 FA11 SAS 11• Remember: formula with _______ integer subscripts (not necessarily the same as the molecular formula!!)• Can determine from percent compositionEXAMPLEbutyric acid has % composition of%C = 54.2% %O = 36.6% %H = 9.15%moles C = (54.2)moles H = (9.15)moles O = (36.6)1 mol C12.0 g1 mol H1.008 g= 4.52= 9.081 mol O16.0 g= 2.29Determination of Molecular FormulaCH110 FA11 SAS 12• To continue and calculate the molecular formula, need more information• Need to know molecular weightEXAMPLE cont.MW butyric acid is 88.1 g/molempirical formula C2H4Oso we must ______ the subscriptsMolecular FormulaDetermination of Empirical Formula by Combustion AnalysisCH110 FA11 SAS 13Problem Solving Strategy1. What do we need to find the empirical formula?2.How can we calculate ratio of moles C to moles H?3. Oxygen is in excess, and other elements are not analyzed for. How can we find the mass of elements other than C and H in the molecule?4. How can we calculate moles from mass?5. What is the empirical formula?Combustion Analysis ExampleCH110 FA11 SAS 14Combustion of 3.14 g isopropyl alcohol (rubbing alcohol) produced 6.90 g CO2and 3.76 g H2O. What is isopropyl alcohol’s empirical formula?Alcohols are composed of carbon, hydrogen, and oxygen. This makes them good examples for combustion! (We will learn more about their properties later.)1) Consider C first:2) Now consider H:Combustion Analysis ExampleCH110 FA11 SAS 153) Now consider O:4) Find ratios of moles of atoms in sample:So far: mols C =mols H =How does this work?CH110 FA11 SAS 16Stoichiometry• Mass is conserved: • Coefficients are the ratio of ______ of reactant to ______ of productOur balanced combustion reaction:Check our work: Conservation of massConservation of atoms works too, but is a lot more painful…Practice ProblemCH110 FA11 SAS 17An analysis of nicotine, an addictive compound found in tobacco leaves, shows that it is 74.0% C, 8.65% H, and 17.35% N. Its molar mass is 162 g/mol. What are its empirical and molecular formulas?Stoichiometry StrategyCH110 FA11 SAS 18If you are given an empirical formula or a percent composition:• Calculate Molecular Formula from empirical formula• Compare FW of sample to FW of empirical formula• What integer relates them to each
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