Chemistry Exam - 1. a. An unknown compound is found to...

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1. a. An unknown compound is found to contain only carbon, hydrogen and oxygen. Complete combustion of a 10.38 mg sample of the compound yielded 16.01 mg of CO 2 and 4.37 mg of H 2 O. The molar mass of the compound is 176.1 g/mol. i. How many mg of each of C and H were in the original sample? 4.37 mg C, 0.49 mg H ii. What is the % w/w of each of C, H and O in the compound? 42.10% C, 4.72% H, 53.18% O ii. What are the empirical and molecular formula of the compound? C 2 H 3 O 2 empirical formula, C 6 H 9 O 6 molecular formula b. The mass spectrum of an unknown ketone showed the molecular ion peak at m/z = 86. The relative abundances of the M to the M+1 peak were in the ratio of about 100 to 5. The other main peaks appeared at m/z = 71 and 43. Suggest a possible structure for this ketone and briefly explain your reasoning. pentan-2-one 2. a. A sample of a mixture of KClO 3 and KBr weighing 7.0950 g is heated in the presence of a small amount of MnO 2 which acts as a catalyst of the decomposition reaction 2 KClO 3(s) ÷ 3O 2(g) + 2KCl (s) . The O 2(g) is collected in a container over water. The total volume and pressure of the gas in the container was 865.5 mL and 741.5 mm Hg respectively at 27.2 o C. The vapour pressure of water at that temperature is 27.1 mm Hg. Assume that all of the KClO 3(s) decomposed. What was the %w/w of KClO 3(s) in the original mixture? 38.01% b. Briefly describe the role of a catalyst in a reaction. Give two other specific examples of reactions and their catalysts. 3. In a laboratory experiment, 0.03404 g of Mg was completely reacted with dilute, aqueous HCl generating H 2(g). The H 2(g) was collected in a gas burette over the aqueous solution at 26.4 o C. The final level of aqueous solution in the burette was 5.00 cm higher than the level in the beaker. The vapour pressure of the aqueous solution at 26.4 o C was 25.8 mm Hg and the barometric pressure that day was 764.8 mm Hg. Given that the density of the aqueous solution is 1.00 g/mL and the density of Hg is 13.6 g/mL, calculate the volume of the H 2(g) that was collected. (atomic mass of Mg = 24.30 g/mol) V = 0.0356 L 4. Calculate the empirical formula of hydrated ferrous ammonium sulfate, Fe a (NH 4 ) b (SO 4 ) c (H 2 O) d , from the following data. a. 0.7840 g of the salt gives 0.1600 g Fe 2 O 3(s) when heated strongly in air to constant mass. b. 0.7840 g of the salt dissolved in water gives 0.9336 g BaSO 4 (s) when excess BaCl 2(aq) is added. c. When 0.3920 g of the salt is dissolved in water and boiled with excess NaOH(aq), NH 3(g) is liberated. When this gas is absorbed in 50.0 mL of 0.10 M HCl(aq), the excess acid remaining after reaction with the NH 3(g) requires 30.0 mL of 0.10 M NaOH for neutralization. (a=1, b=2, c=2, d=6)
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This note was uploaded on 03/13/2010 for the course CHM 90210 taught by Professor Durkka during the Spring '10 term at École Normale Supérieure.

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Chemistry Exam - 1. a. An unknown compound is found to...

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