Chapter 3 Chemical Formulas and Composition of Compounds Prepare for Recitation September 29th ALEKS Objective 5 due September 27th ANGEL Quiz 5 September 29th Lecture 13 September 21st Lecture 14 September 23rd Lecture 15 September 26th Read Ch 2 6 3 3 3 5 9 1 9 3 Additional Preparation BLB 3 25 29 35 37 51 53 Packet 3 1 9 BLB 9 23 25 27 38 Packet 9 1 7 CH110 FA11 SAS 1 Chapter 3 Chemical Formulas and Composition of Compounds KEY QUESTIONS THE BIG PICTURE What 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 By the end of Today s Lecture you should know Relationship between molecular and empirical formula Combustion Analysis Formula and molecular weights Stoichiometry problems CH110 FA11 SAS 2 Exam 2 Concepts You Should Know Chapter 3 Chemical Formulas Again How do we determine the composition of a compound from the various chemical formulas What is combustion analysis Chapter 25 Organic Molecules What are the properties of hydrocarbons What do the functional groups we learned about look like and how do they affect properties Chapter 9 Bonding and Geometry 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 Chapter 11 Intermolecular Forces What are the IMFs we have learned about How do IMFs impact boiling and melting points CH110 FA11 SAS 3 Chemical Formula Chemical Formula Subscripts denote the number of atoms per molecule O2 O3 CH110 FA11 SAS 4 Use of Chemical Formulas Nomenclature Subscripts Coefficients the numbers that come before the formula Conservation of Mass 2H2 O2 2H2O Balance 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 CH110 FA11 SAS 5 Types of Chemical Formulas Molecular Formula H2O H2O2 O2 CH3CH2OH Empirical Formula Structural Formula CH110 FA11 SAS 6 Molecular Weight aka Formula Weight Formula Weight MgCO3 Atom Mg of atoms 1 atomic weight 24 3 C 1 12 0 12 0 O 3 16 0 48 0 Total MgCO3 formula weight molar mass weight 24 3 84 3 amu 84 3 amu 84 3 g mol Molar Mass CH110 FA11 SAS 7 Working with Formulas How many atoms are in the chemical formula Mg3 PO4 2 a 3 b 5 c 10 d 13 How many moles of Mg3 PO4 2 are in a 10 0 g sample a 0 038 moles b 0 059 moles c 0 043 moles How many atoms are in 10 0 g of Mg3 PO4 2 a 2 59 x 1022 atoms b 3 58 x 1022 atoms c 2 29 x 1022 atoms d 2 97 x 1023 atoms CH110 FA11 SAS 8 Empirical Formula Empirical Formula a formula showing the relative abundance of atoms in a compound using the smallest possible whole number ratios How do we find the molecular formula from experimental data Relative ratio from experiment Weight of whole molecule to compare w empirical formula CH110 FA11 SAS 9 Percent Composition The connection between empirical formula and molecular formula of element atoms atomic weight x100 FW of compound What is the mass percent of each element in H3PO4 H 1 008 amu P 30 97 amu O 16 00 amu P O 30 97 x 100 4 16 00 x 100 H 100 CH110 FA11 SAS 10 Determination of Empirical Formula Remember formula with integer subscripts not necessarily the same as the molecular formula Can determine from percent composition EXAMPLE butyric acid has composition of C 54 2 O 36 6 H 9 15 1 mol C 4 52 moles C 54 2 12 0 g 1 mol H 9 08 moles H 9 15 1 008 g moles O 36 6 1 mol O 2 29 16 0 g CH110 FA11 SAS 11 Determination of Molecular Formula To continue and calculate the molecular formula need more information Need to know molecular weight EXAMPLE cont MW butyric acid is 88 1 g mol empirical formula C2H4O so we must the subscripts Molecular Formula CH110 FA11 SAS 12 Determination of Empirical Formula by Combustion Analysis Problem Solving Strategy 1 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 CH110 FA11 SAS 13 Combustion Analysis Example Combustion of 3 14 g isopropyl alcohol rubbing alcohol produced 6 90 g CO2 and 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 CH110 FA11 SAS 14 Combustion Analysis Example So far mols C mols H 3 Now consider O 4 Find ratios of moles of atoms in sample CH110 FA11 SAS 15 How does this work Stoichiometry Mass is conserved Coefficients are the ratio of of reactant to of product Our balanced combustion reaction Check our work Conservation of mass Conservation of atoms works too but is a lot more painful CH110 FA11 SAS 16 Practice Problem An 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 CH110 FA11 SAS 17 Stoichiometry Strategy If 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 other Multiply the subscript of each atom in the empirical formula by that integer If you are given information on a reaction combustion analysis or otherwise Always identify the balanced chemical reaction first Connect moles to mass or mass to moles depending on given information and remember mass is conserved CH110 FA11 SAS 18 What You Should Know Chemical Formulas What do subscripts mean How is this different from a coefficient in a chemical reaction How do you calculate the number of atoms in a molecule What is a molecular formula What is a structural formula What is an empirical formula How is the empirical formula related to the molecular formula Molecular Weight What is the molecular weight or formula weight How do you calculate formula weight How do you use the molecular weight to convert the mass of a sample into moles of molecules Or moles into mass CH110 FA11 SAS 19 What You
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