1 Molecules and Compounds Read Chapter 3. Study all examples and complete all exercises. Complete all bold numbered problems. 2 Chapter 3 Outline • Molecular Formula • Molar Mass • Empirical and Molecular Formula • Nomenclature 3 Compounds & Molecules Buckyball, C60 NaCl, salt 4 Compounds & Molecules • COMPOUNDS are a combination of 2 or more elements in definite ratios by mass. • The character of each element is lost when forming a compound. • MOLECULES are the smallest unit of a compound that retains the characteristics of the compound. 5 MOLECULAR FORMULAS • Formula for glycine is C2H5NO2 • In one molecule there are – 2 C atoms – 5 H atoms – 1 N atom – 2 O atoms 6 WRITING FORMULAS • Formula HOCH2CH2OH to show atom ordering • or in the form of a structural formula 7 Molecular Modeling 8 Molecular Modeling H H O H N C C O H H Ball & stick Drawing of glycine Space-filling 9 Resources for Molecular Modeling • Oxford Molecular/CAChe Scientific software on Saunders General Chemistry CD-ROM • Rasmol and Chime on the Internet • See http://www.saundercollege.com 10 ELEMENTS THAT EXIST AS MOLECULES Allotrope of C Buckyball, C60 11 12 IONS AND IONIC COMPOUNDS • IONS are atoms or groups of atoms with a positive or negative charge. • Taking away an electron from an atom gives a CATION with a positive charge. • Adding an electron to an atom gives an ANION with a negative charge. Formation of Cations & Anions 13 Formation of Cations & Anions Mg --> Mg2+ + 2 e- F + e- --> F- A cation forms when an atom loses one or more electrons. An anion forms when an atom gains one or more electrons 14 15 PREDICTING ION CHARGES MONATOMIC IONS In general • metals (Mg) lose electrons ---> cations • nonmetals (F) gain electrons ---> anions Li1+ F1- LiF 16 Figure 3.7 Also Sn4+ Pb4+ 17 METALS M ---> n e- + Mn+ where n = periodic group Na+ Mg2+ Al3+ Transition metals --> M2+ or M3+ are most common 18 NONMETALS NONMETAL + n e- ------> Xnwhere n = 8 - Group number C4N3O2F- carbide nitride oxide fluoride Bromine POLYATOMIC IONS 19 Groups of atoms with a charge. (See back of periodic chart) 20 Some Common Polyatomic Ions HNO3 nitric acid NO3nitrate ion 21 Some Common Polyatomic Ions NH4+ ammonium ion One of the few common polyatomic cations Some Common Polyatomic Ions CO32carbonate ion HCO3bicarbonate ion hydrogen carbonate 22 Some Common Polyatomic Ions PO43phosphate ion CH3CO2acetate ion 23 Some Common Polyatomic Ions SO42Sulfate ion SO32Sulfite ion 24 25 Some Common Polyatomic Ions NO3Nitrate ion NO2Nitrite ion COMPOUNDS FORMED FROM IONS 26 CATION + ANION COMPOUND Na+ + Cl- A neutral compound requires equal number of + and - charges. NaCl IONIC COMPOUNDS ammonium chloride, NH4Cl NH4+ Cl- 27 28 Some Ionic Compounds Ca2+ + 2 FCaF2 Calcium fluoride Mg2+ + 2 NO3- Mg(NO3)2 Magnesium nitrate Calcium Fluoride 3 Fe2+ + 2 PO43Iron(II) phosphate Fe3(PO4)2 Sample Questions • Predict the charges for the ions formed from: Se P Ga Sr • Give the formula for each ion in Al2(SO4)3 • Give the formula for the ionic compound that forms between Na and S Ga and O Ba and N Answers 29 30 Properties of Ionic Compounds Forming NaCl from Na and Cl2 • A metal atom can transfer an electron to a nonmetal. • The resulting cation and anion are attracted to each other by electrostatic forces. 30 Electrostatic Forces 31 The oppositely charged ions in ionic compounds are attracted to one another by ELECTROSTATIC FORCES. These forces are governed by COULOMB’S LAW. Electrostatic Forces 32 COULOMB’S LAW Force of attraction = (charge on+)(charge on-) (distance between ions)2 As ion charge increases, the attractive force _______________. As the distance between ions increases, the attractive force ________________. This idea is important and will come up many times in future discussions! Importance of Coulomb’s Law NaCl, Na+ and Cl-, m.p. 801 oC MgO, Mg2+ and O2m.p. 2800 oC AlN, Al3+ and N3- m.p. 2900 oC 33 34 Names of Compounds • Rules for nomenclature are found in section 3.5 • STUDY them carefully!! • We will be studying nomenclature in the laboratory in Experiment MA. 35 Counting Atoms • Mg burns in air (O2) to produce white magnesium oxide, MgO. • How can we figure out how much oxide is produced from a given mass of Mg? 36 Counting Atoms • Chemistry is a quantitative science — we need a “counting unit.” • The MOLE • 1 mole is the amount of substance that contains as many particles (atoms, molecules) as there are in 12.0 g of 12C. 37 Particles in a Mole Avogadro’s Number 6.02 x 1023 Amedeo Avogadro 1776-1856 There is Avogadro’s number of particles in a mole of any substance. 38 Mole in Chemistry is NOT: •An informer / spy •Dark spot on Cindy Crawford’s upper lip •Rodent that burrows in the ground •A tunneling machine •Wave break •Spicy Mexican sauce A mole is a convenient measuring tool. 39 Pair Dozen Baker’s Dozen Ream Baseballs 2 baseballs 12 baseballs 13 baseballs 500 baseballs Pineapples 2 Pineapples 12 Pineapples 13 Pineapples 500 Pineapples Calculators 2 Calculators 12 Calculators 13 Calculators 500 Calculators Planets 2 Planets 12 Planets 13 Planets 500 Planets 40 Mole = mol = 23 6.022 10 “particles” Mole Baseballs 6.022 1023 baseballs Pineapples 6.022 1023 Pineapples Calculators 6.022 1023 Calculators Planets 6.022 1023 Planets 41 Just as 1 doz eggs = 12 eggs or 1 mol of eggs = 6.022 1023 eggs A mole is a “Number” 1 mol of H = 6.022 1023 atoms of H 1 mol of O = 6.022 1023 atoms of O 1 mol of Al = 6.022 1023 atoms of Al 1 mol of Cr = 6.022 1023 atoms of Cr 42 In chemistry, the mol is more then just a number 1 mol amu = 1.00 g 1 12 24 1amu mass C 1.661 10 g 12 43 1 mol amu = 1.00 g Proof: 1 mol = 6.022 1023 amu = 1.661 10-24 g 1 mol amu = (6.022 1023) (1.661 10-24g) = 1.00 g Molar Mass 1 mol of 12C = 12.00 g of C = 6.02 x 1023 atoms of C 12.00 g of 12C is its MOLAR MASS Taking into account all of the isotopes of C, the molar mass of C is 12.011 g/mol 44 45 Molar Mass 1 mol of 12C = 12.00 g of C = 6.02 x 1023 atoms of C 12.00 g of 12C is its MOLAR MASS Taking into account all of the isotopes of C, the molar mass of C is 12.011 g/mol Find molar mass from periodic table 13 Al 26.9815 atomic number symbol atomic weight 46 PROBLEM: How many moles are represented by 0.200 g of Mg? 0.200 g 1 mole 24.3 g = 0.00823 mole How many atoms in this piece of Mg? 0.00823 mole 6.02 x 1023 atom 1 mole = 4.95 x 1021 atom 47 MOLECULAR WEIGHT AND MOLAR MASS Molecular weight is the sum of the atomic weights of all atoms in the molecule. Molar mass = molecular weight in grams 48 What is the molar mass of ethanol, C2H6O? 1 mol contains 2 mol C (12.0 g C/1 mol) = 24.0 g C 6 mol H (1.0 g H/1 mol) = 6.0 g H 1 mol O (16.0 g O/1 mol) = 16.0 g O TOTAL = molar mass = 46.0 g/mol 49 Tylenol • Formula = C8H9NO2 • Molar mass = 151.0 g/mol 50 How many moles of alcohol are there in a “standard” can of beer if there are 21.3 g of C2H6O? 21.3 g 1 mole 46.0 g = 0.463 mole 51 How many molecules of alcohol are there in a “standard” can of beer if there are 21.3 g of C2H6O? 0.463 mole 6.02 x 1023 molecule 1 mole = 2.79 x 1023 molecule 52 How many atoms of C are there in a “standard” can of beer if there are 21.3 g of C2H6O? 2.79 x 1023 molecule 2 atom C 1 molecule = 5.58 x 1023 atom C Sample problems Empirical and Molecular Formulas A pure compound always consists of the same elements combined in the same proportions by weight. Therefore, we can express molecular composition as PERCENT BY WEIGHT Ethanol, C2H6O 52.2% C, 13% H, 34.8% O 53 Percent Composition 54 Consider some of the family of nitrogen-oxygen compounds: NO2, nitrogen dioxide and closely related, NO, nitrogen monoxide (or nitric oxide) Structure of NO2 Chemistry of NO, nitrogen monoxide (nitric oxide) Percent Composition 55 Consider NO2, Molar mass = ? What is the weight percent of N and of O? Wt . % N = 14.0 g N 46.0 g NO 2 • 100% = 30.4 % (16 .0 g O per mole ) 2 x 100 % = 69 .6% Wt. % O = 46 .0 g Percent Composition 56 What are the weight percentages of N and O in NO? 14.0 g N Wt. % N = 30.0 g NO Wt. % O = 16.0 g O 30.0 g NO • 100% = 46.7 % • 100%= 53.3% Percent Composition Samples Problems 57 1. Calculate the percent composition of H2O. 2.0 g H Wt. % H = • 100% = 11 % 18.0 g H2O gO 16.0 Wt. %O = 18.0 g H2O •100%=88.9% 58 Percent Composition Samples Problems 2. Calculate the percent O in NaOH. 16.0 g O Wt. %O •100% 40.0% 40.0 g NaOH 59 Percent Composition Samples Problems 3. Calculate the percent O and the percent water in CuSO4.5H2O. gO 144.0 Wt. % O = • 100% = 57.69 % 249.6 g CuSO4 • 5H2O 90.0 g H2O • 100% = 36.1 % Wt. % H2O = 249.6 g CuSO4.5H2O 60 Determining Formulas In chemical analysis we determine the % by weight of each element in a given amount of pure compound and derive the EMPIRICAL or SIMPLEST formula. A compound of B and H is 81.10% B. What is its empirical formula? B 81.10 g H 1 mole 18.90 g 10.8 g 1 mole 1.0 g 7.51 mole B 19 mole H 7.51 mole B 7.51 mole B 1.00 mole B 2.00 mole B 2.5 mole H 5.0 mole H B2H5 61 62 A compound of B and H is 81.10% B. Its empirical formula is B2H5. What is its molecular formula? Is the molecular formula B2H5, B4H10, B6H15, B8H20, etc.? B2H6 B2H6 is one example of this class of compounds. A compound of B and H is 81.10% B. Its empirical formula is B2H5. What is its molecular formula? We need to do an EXPERIMENT to find the MOLAR MASS. Here experiment gives 53.3 g/mol. Compare with the mass of B2H5 , 26.66 g/unit Find the ratio of these masses. 2 units of B2 H 5 53.3 g/mol = 26.66g/unit of B2 H5 1 mol Molecular formula = B4H10 63 64 Determine the formula of a compound of Sn and I using the following data. • Reaction of Sn and I2 is done using excess Sn. • Mass of Sn in the beginning = 1.056 g • Mass of iodine (I2) used = 1.947 g • Mass of Sn remaining = 0.601 g 65 Tin and Iodine Compound Find the mass of Sn that combined with 1.947 g I2. Mass of Sn initially = 1.056 g Mass of Sn recovered = 0.601 g Mass of Sn used = 0.455 g Tin and Iodine Compound Sn 0.455 g 1 mole I 1.947 g 118.7 g 1 mole 126.9 g 0.0383 mole Sn 0.1534 mole I 0.0383 mole Sn 0.0383 mole Sn 1.00 mole Sn 4.01 mole I SnI4 66 67 More Problems 2. Calculate the formula for the iron sulfide that forms when 53.73g Fe react with 46.27 g of sulfur. Fe S 53.73 g 1 mole 46.27 g 1 mole 55.8 g 32.1 g 0.963 1.44 0.963 0.963 1.00 1.50 2.00 3.00 Fe2S3 More Problems Calculate the empirical formula a compound containing 90.7% Pb and 9.33% O. Pb 1 mole 90.7 g O 9.33 g 207.2 g 1 mole 16.0 g 0.438 0.583 0.438 0.438 1.00 1.33 3.00 3.99 Pb3O4 68 69 More Problems Calculate the empirical formula for a compound containing 36.5% Na, 25.4% S and 38.1% O. Na S O 36.5 g 1 mole 25.4 g 1 mole 38.1 g 1 mole 23.0 g 32.1 g 16.0 g 1.59 0.791 2.38 0.791 0.791 0.791 2.01 1.00 3.01 Na2SO3 70 More Problems Calculate the empirical and molecular formulas for nicotine, 74.0% C, 8.7% H and 17.3% N, with a molar mass of 160 g/mole. C H N 74.0 g 1 mole 12.0 g 8.7 g 1 mole 17.3 g 1 mole 1.0 g 14.0 g 6.17 8.7 1.24 1.24 1.24 1.24 4.98 7.0 1.00 Empirical formula C5H7N More Problems 7. Calculate the empirical and molecular formulas for nicotine, 74.0% C, 8.7% H and 17.3% N, with a molar mass of 160 g/mole. Empirical formula 160 C5H7N =2 81 Molecular formula C10H14N2 71 72 Practice Problems Names/Formulas FeO Pb(C2H3O2)2 magnesium bromide sodium chromate calcium phosphate ammonium carbonate 73 Practice Problems (NH4)2S As2O3 SO2 silicon disulfide As2S5 dinitrogen monoxide 74 Practice Problems As2S3 dinitrogen pentoxide silicon tetrabromide diphosphorus pentoxide HBrO3 H3PO4(aq) H2CO3 75 Practice Problems Calculations 1. 26 g H2 is how many moles H2? 2. 4.25 x 1021 molecules NH3 is how many grams NH3? 3. 1.5 x 102 formula units KClO3 is how many moles KClO3 4. 0.0042 mole Fe2O3 is how many formula units Fe2O3? 5. 2.15 moles MgSO4.7H2O is how many g MgSO4.7H2O? 76 Practice Problems 6. 25 molecules HBr is how many mole HBr? 7. .00002 g Sn is how many atoms Sn? 8. 7.25 mole H2S is how many g H2S? 9. 5.2 g Sr(OH)2is how many formula units Sr(OH)2? 10. How many moles of CCl4 will contain 2.4 g of chlorine? 77 Practice Problems 11. 19 g of HNO3 contains how many a) molecules of HNO3? b) grams of O? 12. Calculate the percent composition of NaCl. 13. Calculate the empirical and molecular formulas for a compound containing 43.7g P and 56.3g O, with a molar mass of 140 g/mole. 78 Practice Problems Answers Iron(II) oxide, ferrous oxide lead(II) acetate, plumbous acetate MgBr2 Na2CrO4 Ca3(PO4)2 (NH4)2CO3 ammonium sulfide diarsenic trioxide sulfur dioxide SiS2 As2S5 N2O diarsenic trisulfide N2O5 SiBr4 P2O5 bromic acid phosphoric acid carbonic acid 79 Practice Problems Answers 1. 13 mole 2. 0.120 g 3. 2.5 x 10-22 mole 4. 2.5 x 1021 atom 5. 530. g 6. 4.2 x 10-23 mole 7. 1 x 1017 atom 8. 247 g 9. 2.6 x 1022 formula units 10. 0.017 mole 11. a) 1.8 x 1023 molecule b) 14 g 12. 39.3%, 60.7% 13. C5H7N 14. P2O5 80 Sample Questions • Predict the charges for the ions formed from: Se P -2 -3 Ga Sr +3 +2 Al3+ SO42- • Give the formula for each ion in Al2(SO4)3 • Give the formula for the ionic compound that forms between Na and S Ga and O Ba and N Na2S Ga2O3 Ba3N2 81 Sample Problems 1. 2.5 mole S = ? atom S 2.5 mole 6.02 x 1023 atom 1 mole = 1.5 x 1024 atom S 82 Sample Problems 2. 2.1 mole Zn = ? g Zn 2.1 mole 65.4 g 1 mole = 140 g Zn 83 Sample Problems 3. 1.42 g Mg = ? atom Mg 1.42 g 1 mole 6.02 x 1023 atom 24.3 g 1 mole = 3.55 x 1022 atom 84 Sample Problems 4. 125.2 g O2 = ? mole O2 125.2 g 1 mole 32.0 g = 3.91 mole O2 85 Sample Problems 1. 1.5 mole H2O = ? g H2O 1.5 mole 18.0 g 1 mole = 27 g H20 86 Sample Problems 2. 1.50 mole CCl4 = ? molecules CCl4 1.5 mole 6.02 x 1023 molecule 1 mole = 9.03 x 1023 molecule 87 Sample Problems 3. 1.25 mole CaCl2 = ? formula units CaCl2 1.25 mole 6.02 x 1023 formula unit 1 mole = 7.52 x 1023 formula unit 88 Sample Problems 4. 2.5 g K = ? g KOH 2.5 g K 1 mole K 39.1 g K 1 mole KOH 56.1 g KOH 1 mole K 1 mole KOH = 3.6 g KOH 89 Sample Problems 5. 34.5 g CaCO3 = ? g O 34.5gCaCO3 1 moleCaCO3 3 moleO 16.0 gO 100.1gCaCO3 1 moleCaCO3 1 moleO = 16.5 g O