1 Chemical Formulas: Molecular vs. Empirical Molecular formula - chemical formula with subscripts that give actual whole # ratio of elements in a compound. Molecular formula is a whole # multiple of the empirical formula. Empirical formula – chemical formula with subscripts that give the smallest whole # ratio of elements in a compound. Compound Molec. Formula Emp. Formula acetylene C2H2 CH ethane C2H6 CH3 benzene C6H6 CH butane C4H10 C2H5 Note: Ionic compounds only have empirical formulas due to the definition of the formula unit. Calculation of Empirical Formula from Wt% and Mass Data Step 1: Find masses of all elements present. Step 2: Convert masses to moles. Step 3: Divide by smallest # moles found to obtain mole ratios. Step 4: If mole ratios are close enough to whole numbers, round, and use these as subscripts in empirical formula. Step 5: If mole ratios are NOT close enough to whole numbers, multiply by a small whole # to obtain whole numbers.2 Examples: Wt % Data In solution, a student reacted ruthenium(III) chloride and lithium sulfide. The black precipitate obtained was heated in H2S. After heating, the black precipitate was found to contain 61.18% Ru and 38.82%S. What was the empirical formula of the black precipitate? Mass Data A 0.8684 g sample of sodium pertechnetate was analyzed and found to contain 0.1114 g Na, 0.4562 g Tc, and oxygen. What is the empirical formula of sodium pertechnetate? Empirical Formulas From Reaction Data Combustion Analysis Data A 5.048 g sample of a compound containing only C, H, and O was placed in a combustion furnace. Combustion of the sample yielded 7.406 g CO2 and 3.027 g H2O. Calculate the empirical formula of this compound. NOTE: All C from compound ends up in CO2 and all H in H2O. Step 1: Calculate mass C from CO2 produced. Step 2: Calculate mass H from H2O produced. Step 3: Calculate mass “O” from law of conservation of mass. Step 4: Find empirical formula.3 Obtaining the Molecular Formula Two pieces of information are needed: 1. Empirical formula 2. Molar mass Remember…..molecular formula is whole # multiple of empirical formula. Ex. CxHy (x n) = CnxHny To find the molecular formula, it is necessary to find the whole #!! Whole # = Molar Mass Empirical Mass Examples: A compound has an empirical formula of C10H21 and a molar mass of 282.54. What is the molecular formula of this compound? Learning Check A compound has the empirical formula CH. Which of the following molar masses are not possible for this compound: 26.0, 32.6, 39.1, 52.1, and 61.8?4 • Chemical and Physical Rxns • Rounding, SigFigs, and Scientific Notation • Make sure you know the rules for +, - vs. x, ÷ • Factor Label Method and Conversions • Memorize most common metric (i.e. kg, g, cg, mg, μg, ng) • All other conversions given • Density calculations • Atomic Theory/History • Dalton, Thompson, Milliken, Rutherford • Subatomic Particles • Characteristics of each and location within the atom • Atomic number and atomic mass, ions • Calculating and using Relative Atomic Mass • Isotopes • Be able to calculate the average atomic mass from % abundance and visa versa • Periodic Table Trends Exam 1 Material • Diatomic Molecules • Chemical Compounds and Formula Units • Polyatomic Ions • Memorize names and charges • Molecular vs. Ionic Compounds • Nomenclature • Ionic Nonvariable (IA, IIA, and Aluminum) = metal + nm-ide • Ionic Variable (transition elements) – stock sys. = Roman numerals for charge *(Hg22+ exception) + nm-ide – old sys. = Latin name (ferrum, cuprum, etc) + -ous or -ic +nm-ide • Molecular – First element + second element-ide – Greek prefixes to specify # • Acids – Binary acids = hydro-nm-ic – Oxyacids = suffixes –ous and –ic, prefixes hypo- and per- • Polyatomic Ions • Moles • Avogadros number • % Composition • Empirical and Molecular formulas Exam 1 Review