16Ch9part1ST - Molecular Weight Chemical Formulas What do...

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Unformatted text preview: Molecular Weight Chemical Formulas What do subscripts mean? How is this different from a coefficient in a chemical reaction? 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 it related to the molecular formula? Why is an empirical formula important? LRSVDS CH110 Molec. Geometry 1 LRSVDS What is atomic weight? What is molecular weight or formula weight (FW)? How can you calculate FW of a given molecule using the periodic table? Use the molecular weight to convert the mass of a sample into moles of molecules. Use the molecular weight to convert moles into mass of molecules. Convert moles of molecules into number of atoms using Avogadro's number. CH110 Molec. Geometry 2 Percent Composition, Mass % Calculate mass percent of an element in a molecule; know the ratio you must use. Convert mass percent to grams, assuming you have 100 g of sample. Given the weight of a sample, use the mass percent to calculate the mass of an element in the sample. Given the mass percent, convert this to number of moles of each element in the sample, assuming 100 g of sample. Empirical and Molecular Formula Given percentages of elements in a sample, determine the empirical formula using mole ratios. Given the FW of a compound, convert the empirical formula to the molecular formula. Find the integer that relates the two molecular weights (calculated and experimental) Multiply the empirical formula by the integer to get molecular formula CH110 Molec. Geometry 4 LRSVDS CH110 Molec. Geometry 3 LRSVDS Combustion Analysis Given the mass of CO2 and H2O produced, find the number of moles of C and moles of H in the original sample. Find the mole ratio of C:H. If another element is present (like O or N), find the moles of that element in the original sample by: Calculating the mass of C and mass of H in the original sample using the number of moles and atomic weight. Subtract the mass of C and the mass of H from the weight of the original sample to get mass of the extra element. Convert mass of the extra element to moles, and find the mole ratio compared to the moles of C and H. Lecture 16: Molecular Geometry Read: HW: BLB 9.19.2 BLB 9.15,17,21,25,27 Sup 9:17 Know: VSEPR theory electron pair (electron domain) geometry molecular geometry molecules with more than one central atom molecules with multiple bonds SCT #6 Nomenclature: Bonus Deadline Thurs. 2/21 at midnight Write the empirical formula using the ratios. Write the molecular formula if given the molecular weight of the original compound. LRSVDS CH110 Molec. Geometry 5 Exam #2: Monday, March 3 @ 6:30 PM review previous material UNDERSTAND what we've done start preparing now! LRSVDS CH110 Molec. Geometry 6 What Determines the Shape of a Molecule? Structural Formulas tell us how atoms are connected to each other Lewis structures tell us where the electrons are shape reactivity Predict the shape of the molecule by counting: 1. 2. Electron pairs ______ each other. Place the electron pairs as far as possible from each other predict the shape of the molecule. Valence Shell Electron Pair Repulsion Theory (VSEPR) LRSVDS CH110 Molec. Geometry 7 LRSVDS CH110 Molec. Geometry 8 Electron Domains Electron pairs Electron domains One lone pair = one electron domain One bond (single, double or triple bond) = one electron domain. Electron-Domain Geometries How many electron domains around A? LRSVDS CH110 Molec. Geometry 9 LRSVDS CH110 Molec. Geometry 10 Electron-Domain Geometries Count the number of __________ in the Lewis structure. The geometry corresponds to that number of electron domains. Molecular Geometries Is the electron-domain geometry always the shape of the molecule? The molecular geometry is defined by the positions of only the atoms in the molecules, not the nonbonding pairs. LRSVDS CH110 Molec. Geometry 11 LRSVDS CH110 Molec. Geometry 12 Molecular Geometries Some electron domains have more than one molecular geometry. Linear Electron Domain In this domain, there is only one molecular geometry: linear. NOTE: If there are only two atoms in the molecule, the molecule will be linear no matter what the electron domain is. LRSVDS CH110 Molec. Geometry 13 LRSVDS CH110 Molec. Geometry 14 Trigonal Planar Electron Domain Lone Pairs and Bond Angles Nonbonding pairs are physically larger than bonding pairs. Therefore, their repulsions are greater; this tends to _________ bond angles on the other side of the molecule. Two molecular geometries: Trigonal planar, if all the electron domains are bonding Bent, if one of the domains is a nonbonding (lone) pair. LRSVDS CH110 Molec. Geometry 15 LRSVDS CH110 Molec. Geometry 16 Multiple Bonds and Bond Angles Tetrahedral Electron Domain Double/triple bonds: greater electron density on one side of the central atom bond angles involving multiple bond are ___________, angles on other side of the molecule are ___________. LRSVDS CH110 Molec. Geometry 17 Three molecular geometries: Tetrahedral, if all are ______ pairs Trigonal pyramidal if one is a ______________ Bent if there are two ____________ LRSVDS CH110 Molec. Geometry 18 Trigonal Bipyramidal Electron Domain Trigonal Bipyramidal Electron Domain Lone pairs prefer to be equatorial LRSVDS CH110 Molec. Geometry 19 Four distinct molecular geometries : Trigonal bipyramidal Seesaw T-shaped Linear LRSVDS CH110 Molec. Geometry 20 Octahedral Electron Domain How to determine the shape of a molecule 1. Draw the Lewis structure of the molecule. 2. Determine the electron-pair geometry (EPG: count electron domains: multiple bonds count as one domain). 3. Focus on bonded-electron pairs ONLY to determine the molecular geometry (MG) All positions are equivalent Two lone pairs go on ___________ sides of the molecule Three molecular geometries: Octahedral Square pyramidal Square planar LRSVDS CH110 Molec. Geometry 21 Examples: COCl2 PCl5 BrF3 LRSVDS CH110 Molec. Geometry 22 Bond Angles Octahedral Tetrahedral Trigonal Planar Linear 90 109.5 120 180 Larger Molecules Describe geometry around a particular atom Determine the approximate bond angles indicated Angle #1 1. 2. 3. 4. 5. LRSVDS Angle #2 109 120 90 120 109 23 LRSVDS CH110 Molec. Geometry 24 109 180 120 109 120 CH110 Molec. Geometry Molecular Shapes: Summary Number Number of of electron bonded domains atoms Molecular geometry Example Practice Problems CO2 BF3 O3 CH4 NH3 H2O PCl5 SF4 BrF3 XeF2 SF6 BrF5 XeF4 2 3 3 4 4 4 5 5 5 5 6 6 6 2 3 2 4 3 2 5 4 3 2 6 5 4 linear trigonal planar bent tetrahedral trigonal pyramidal bent trigonal bipyramidal seesaw T-shaped linear octahedral square pyramidal square planar Which of the following molecules has four electron domains about the central atom? A. CO2 B. SO3 C. CHCl3 D. SeF4 LRSVDS CH110 Molec. Geometry 25 LRSVDS CH110 Molec. Geometry 26 Practice Problem The molecular geometry of which molecule is square planar? A. CCl4 Practice Problems The molecular geometry of TeF3- is best described as: A. linear B. trigonal planar B. XeF4 C. see-saw C. PH3 D. XeF2 E. ICl3 D. T-shaped E. tetrahedral LRSVDS CH110 Molec. Geometry 27 LRSVDS CH110 Molec. Geometry 28 ...
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This note was uploaded on 07/23/2008 for the course CHEM 110 taught by Professor Hofmann,brucerob during the Spring '08 term at Penn State.

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