Ch3_Formulas - Empirical and Molecular Formulas A pure...

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Unformatted text preview: 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 composition PERCENT Percent Composition Percent Composition Consider some of the family of nitrogenoxygen compounds: NO2, nitrogen dioxide and closely related, NO, nitrogen monoxide (or nitric oxide) Consider some of the family of nitrogenoxygen compounds: NO2, nitrogen dioxide and closely related, NO, nitrogen monoxide (or nitric oxide) WEIGHT Ethanol, C2H6O 52.13% C 13.15% H 13.15% 34.72% O 34.72% Percent Composition Consider NO2, Molar mass = ? What is the weight percent of N and of O? Wt. % N = 14.0 g N • 1 00% = 30.4 % 46.0 g NO 2 Wt. % O = 2 ( 16.0 g O per mole) x 1 00% = 6 9.6% 46.0 g What are the weight percentages of N and O in NO? Structure of NO2 Structure of NO2 Chemistry of NO, nitrogen monoxide 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. SIMPLEST formula. PROBLEM : A compound of B and H is 81.10% B. What is its empirical formula? Page 1 A compound of B and H is 81.10% B. What is its empirical formula? • Because it contains only B and H, it must contain 18.90% H. • In 100.0 g of the compound there are 81.10 g of B and 18.90 g of H. • Calculate the number of moles of each constitutent . A compound of B and H is 81.10% B. What is its empirical formula? Calculate the number of moles of each element in 100.0 g of sample. 1 mol 81.10 g B • = 7 .502 mol B 10.81 g 18.90 g H • 1 mol = 1 8.75 mol H 1.008 g A compound of B and H is 81.10% B. Its empirical formula is B2H5. What is Its empirical its molecular formula? its molecular Is the molecular formula B 2H5, B4H 10, B6H15, B8 H20, etc.? B2H6 A compound of B and H is 81.10% B. What is its empirical formula? Now, recognize that atoms combine in Now, atoms the ratio of small whole numbers. 1 atom B + 3 atoms H --> 1 molecule BH 3 or 1 mol B atoms + 3 mol H atoms ---> 1 mol BH 3 molecules Find the ratio of moles of elements in the compound. A compound of B and H is 81.10% B. Its empirical formula is B2H 5. What is its molecular formula? We need to do an EXPERIMENT to find We EXPERIMENT the MOLAR MASS. Here experiment gives 53.3 g/ mol Compare with the mass of B 2H5 = 26.66 g/unit Find the ratio of these masses. 2 units of B2H5 53.3 g/mol = 1 mol 26.66 g/unit of B2H5 B2H6 is one example of this class of compounds. Molecular formula = B 4H10 Page 2 A compound of B and H is 81.10% B. What is its empirical formula? Take the ratio of moles of B and H. Always Take Always divide by the smaller number. divide 18.75 mol H 2.499 mol H 2.5 mol H = = 1.000 mol B 1.0 mol B 7.502 mol B But we need a whole number ratio. 2.5 mol H/1.0 mol B = 5 mol H to 2 mol B EMPIRICAL FORMULA = B2H5 Determine the formula of a Determine the formula of a compound of Sn and II using the compound of Sn and using the following data. 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 used = 1.947 g • Mass of Sn remaining = 0.601 g 0.601 • See p. 133 Tin and Iodine Compound Tin and Iodine Compound 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 Find moles of Sn used: Now find the number of moles of I 2 that combined with 3.83 x 10 -3 mol Sn . Mass of I2 used was 1.947 g. Now find the ratio of number of moles of moles of I and Sn that combined. 0.455 g Sn • 1 mol = 3.83 x 10-3 mol Sn 118.7 g 1.947 g I2 • 1 mol = 7.671 x 10-3 mol I2 253.81 g This is equivalent to 2 x 7.671 x 10 -3 or 1.534 x 10-2 mol iodine atoms Page 3 1.534 x 10 -2 mol I 3.83 x 10-3 mol Sn = 4.01 mol I 1.00 mol Sn Empirical formula is SnI4 Empirical SnI ...
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This note was uploaded on 10/19/2011 for the course CHM 2210 taught by Professor Staff during the Spring '08 term at University of Central Florida.

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