Week 3 Key Practice Problems

Week 3 Key Practice Problems - CHEMISTRY 201X Practice...

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September 4-7, 2007 Practice Problems (22) (a) Calculate the density (g/mL) of sulfur hexafluoride gas at 720 torr and 35.0°C. MW(SF 6 ) = 146.054 g/mol p = 720 torr = 0.9474 atm T = 308.15 K Density(SF 6 ) = n(MW)/V = p(MW)/RT = (0.9474 atm)(146.054 g/mol) / (0.0821 L atm/mol K)(308.15 K) = 5.469 g/L = 0.00547 g/mL (b) Calculate the molar mass (g/mol) of a vapor that has a density of 7.135 g/L at 12°C and 743 torr. p = 743 torr = 0.9776 atm T = 285.15 K MW(gas) = (Density)RT/p = (7.135 g/L)(0.0821 L atm/mol K)(285.15 K)/0.9776 atm = 170.86 g/mol (23) Calcium hydride, CaH 2 , reacts with water to form hydrogen gas and calcium hydroxide. (a) Write the balanced chemical equation for this reaction. CaH 2 (s) + 2 H 2 O(l) ⎯→ 2 H 2 (g) + Ca(OH) 2 (aq) (b) How many grams of calcium hydride are necessary to generate 60.0 L of hydrogen gas if the pressure of hydrogen is 740.0 torr at 20°C? # moles H 2 = pV/RT = (0.9737 atm)(60.0 L) / (0.0821 L atm/mol K)(293.15 K) = 2.427 moles H 2 # grams CaH 2 = (2.427 mol H 2 )(1 mol CaH 2 /2 mol H 2 )(42.094 g/mol) = 51.08 g (24) A self-contained breathing apparatus used by firefighters and rescue workers rely on the reaction between potassium superoxide, KO 2 (s), and water vapor (in the breath) to generate oxygen gas. (a) Write the balanced chemical equation for this process. Potassium hydroxide is another by-product of the reaction. 4 KO 2 (s) + 2 H 2 O(g) ⎯→ 3 O 2 (g) + 4 KOH(aq) (b) How many grams of potassium superoxide are necessary to produce 20.0 L oxygen gas at normal conditions? # moles O 2 = pV/RT = (1.00 atm)(20.0 L) / (0.0821 L atm/mol K)(298.15 K) = 0.817 mol O 2 # grams KO 2 = (0.817 mol O 2 )(4 mol KO 2 /3 mol O 2 )(71.10 g/mol) = 77.45 g (c) How many grams of potassium hydroxide will be generated when 20.0 L oxygen gas are produced? (Assume 100% yield)
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Week 3 Key Practice Problems - CHEMISTRY 201X Practice...

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