examquestions07-08 - 1 a b 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 22 CO and 4.37 mg of H 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? 232 696 C H O empirical formula, C H O 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 3 2. a. A sample of a mixture of KClO and KBr weighing 7.0950 g is heated in the 2 presence of a small amount of MnO which acts as a catalyst of the decomposition 3(s) 2(g) (s) reaction 2 KClO ÷ 3O + 2KCl . 2(g) The O 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 C. o The vapour pressure of water at that temperature is 27.1 mm Hg. Assume that all 3(s) 3(s) of the KClO decomposed. What was the %w/w of KClO 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 2(g). 2(g) HCl generating H The H was collected in a gas burette over the aqueous solution at 26.4 C. The final level of aqueous solution in the burette was 5.00 cm higher than the o level in the beaker. The vapour pressure of the aqueous solution at 26.4 C was 25.8 mm o 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 2(g) of the H 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, a 4b 4c 2 d Fe (NH ) (SO ) (H O) , from the following data. 2 3(s) a. 0.7840 g of the salt gives 0.1600 g Fe O when heated strongly in air to constant mass. 4 b. 0.7840 g of the salt dissolved in water gives 0.9336 g BaSO (s) when excess 2(aq) BaCl is added. c. When 0.3920 g of the salt is dissolved in water and boiled with excess NaOH(aq), 3(g) NH is liberated. When this gas is absorbed in 50.0 mL of 0.10 M HCl(aq), the
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3(g) excess acid remaining after reaction with the NH requires 30.0 mL of 0.10 M NaOH for neutralization. (a=1, b=2, c=2, d=6) 3(s) 5. M is a metal; X is a non-metal. Upon strong heating of 0.5000g of MXO , it (s) 2(g) (s) decomposes forming MO and XO . Then, all of the MO was dissolved in water and excess acid, 100 mL of 0.120M HCl was added. The resulting solution required 40.0 mL 2(g) of 0.050 M NaOH for complete neutralization. The rate of effusion of the XO was 2(g) found to be 0.797 times that of N at the same temperature.
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This note was uploaded on 12/03/2009 for the course CHM CHM140 taught by Professor Krish during the Spring '09 term at University of Toronto.

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examquestions07-08 - 1 a b An unknown compound is found to...

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