C142A Worksheet 3 - Chemistry 142A Autumn 2011 Worksheet#3 Answer Key Ch 3 start of Ch 4 1 There are several important compounds that contain only

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1 Chemistry 142A, Autumn 2011 Worksheet #3 Answer Key Ch. 3, start of Ch. 4 1) There are several important compounds that contain only nitrogen and oxygen. Calculate the mass percent of nitrogen in each of the following. (Do (b) only.) a) NO, a gas formed by the reaction of N 2 and O 2 in internal combustion engines. b) NO 2 , a brown gas mainly responsible for the brownish color of photochemical smog. c) N 2 O 4 , a colorless liquid used as a fuel in space shuttles. d) N 2 O, a colorless gas used as an anesthetic by dentists (known as laughing gas). 2) Give the empirical formula for each of these compounds. Do(b) only.
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2 a) Blue = Nitrogen, Red = Oxygen. We have 2 N and 4 O. Reducing to the simplest whole-number ratio, we get 2 O for each 1 N. The empirical formula is therefore NO 2 . b) Black = Carbon, White = Hydrogen. We have 3 C and 6 H. Reducing to the simplest whole-number ratio, we get 2 H for each 1 C. The empirical formula is therefore CH 2 . c) Yellow = Phosphorus, Red = Oxygen. We have 4 P and 10 O. Reducing to the simplest whole-number ratio, we get 5 O for each 2 P. The empirical formula is therefore P 2 O 5 . d) Black = Carbon, Red = O, White = H. We have 6 C, 6 O, and 12 H. Reducing to the simplest whole-number ratio, we get 2 H for every 1 C or 1 O. The empirical formula is therefore CH 2 O . 3) Terephthalic acid is an important chemical used in the manufacture of polyesters and plasticizers. It contains only C, H, and O. Combustion of 19.81 mg terephthalic acid produces 41.98 mg CO 2 and 6.45 mg H 2 O. If 0.250 mol of terephthalic acid has a mass of 41.5 g, determine the molecular formula for terphthalic acid. We are told that every molecule of terephthalic acid contains C, H, and O. We are also told that when 19.81 mg of this compound is combusted, the products are 41.98 mg of CO 2 and 6.45 mg of H 2 O. Finally, we are told that 0.250 mol of terephthalic acid has a mass of 41.5 g. The first step in tackling this problem is to realize that if we can deduce what happens during combustion, we can use the known values of the combustion products to figure out the make-up of the starting materials. Combustion occurs when a compound reacts with gaseous oxygen (this is what happens when something burns). For the purposes of this problem, we should assume that the terephthalic acid is combusted completely, and that the only combustion products are the ones mentioned. We should also assume that the acid is combusted in excess oxygen. Terephthalic Acid + O
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This note was uploaded on 01/18/2012 for the course CHEM 142A taught by Professor Campbell during the Fall '11 term at University of Washington.

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C142A Worksheet 3 - Chemistry 142A Autumn 2011 Worksheet#3 Answer Key Ch 3 start of Ch 4 1 There are several important compounds that contain only

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