CH131_2a

CH131_2a - Chapter 2: Chemical formulas, chemical equations...

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Chapter 2: Chemical formulas, chemical equations and Reaction Yields molecular formula => # atoms of each element in a molecule e.g. H 2 S => 2 atoms H, 1 atom S in each molecule Can be defined for all gases, liqs. and solids that have well- defined molecular subunits. However, in some materials interactions bet. mol. subunits (inter) same as intra molecular interactions. => crystals Sodium chloride, Na Cl , crystal structure Can be considered one giant molecule - each Na + (Cl ) surrounded by 8 Cl (Na + ).
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Consequently, we often refer to chemical formula for a crystal by the empiral formula. empiral formula => simplest formula that gives the correct # of relative atoms in a substance. (more general than just crystals) e.g. NaCl (also called the formula unit of a crystal) Note: sometimes (inorganic complexes) distinct molecula species are found in formula unit (often H 2 O); designated by a dot => example: copper sulfate •5H 2 O vs. CuSO 4 waters of hydration anhydrous
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From empirical formula and definition of mole some important mass relationships can be determined. For example: % composition problems Consider the gas, phosgene. An elemental analysis determines phosgene to be composed by mass of: 12.1% C 16.2% O 71.7% Cl What is the empirical formula of phosgene? Need to determine relative # of atoms in compound. Suppose we 100 g phosgene => # mol C = 12.1 g C 1 mol C 12.0 g C = 1.01 mol C # mol O = 16.2 g O 1 mol O 16.0 g C = 1.01 mol O # mol Cl = 71.7 g Cl 1 mol Cl 35.5 g C = 2.02 mol Cl Ans: ratio 1:1:2 Empirical formula COCl 2
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The actual molecular formula for phosgene is of the type: C n O n Cl 2n where n = 1, 2, 3, … You need additional information to determine n, such as molecular mass => 198 amu Weight of empirical formula = 12 + 16 + 2x35.5 = 99 amu So molecular formula is (n = 2) C 2 O 2 Cl 4
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Another type of problem related to empirical formula and mol ideas and % composition problems Combustion analysis problems (combustion => specific type of chemical reaction) A compound containing C and H is burned (combusted) in an excess of O 2 . C and H converted to CO 2 and H 2 O respectively. C => CO 2 and H => H 2 O Amount of CO 2 and H 2 O produced tells us how much C, H in original compound (hydrocarbon if just C and H). Empirical formula determined. Example…. .
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Consider the combustion of ascorbic acid (vitamin C): Observed => 1.000 g ascorbic acid (C, H, O) => 1.500 g CO 2 + 0.405 g H 2 O What is the % composition of ascorbic acid? (relative mass C, H, O) What is empirical formula of ascorbic acid?
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This note was uploaded on 04/17/2008 for the course CAS CH131 taught by Professor Zigler during the Spring '08 term at BU.

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CH131_2a - Chapter 2: Chemical formulas, chemical equations...

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