Exam 3-solutions

# Exam 3-solutions - Version 317 Exam 3 mccord(50970 This...

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Version 317 – Exam 3 – mccord – (50970) 1 This print-out should have 30 questions. Multiple-choice questions may continue on the next column or page – find all choices before answering. McCord CH301 This exam is only for Dr. McCord’s CH301 classes. 001 10.0 points The density of the vapor of allicin, a compo- nent of garlic, is 1 . 14 g · L 1 at 125 C and 175 Torr. What is the molar mass of allicin? 1. 21 . 6 g · mol 1 2. 50 . 8 g · mol 1 3. 273 g · mol 1 4. 869 g · mol 1 5. 162 g · mol 1 correct Explanation: T = 125 C + 273.15 K = 398 . 15 K P = (175 Torr) 1 atm 760 Torr = 0 . 230263 atm ρ = 1 . 14 g / L The ideal gas law is P V = n R T n V = P R T with unit of measure mol/L on each side. Multiplying each by molar mass (MM) gives n V · MM = P R T · MM = ρ , with units of g/L. MM = ρ R T P = (1 . 14 g / L) ( 0 . 08206 L · atm mol · K ) 0 . 230263 atm × (398 . 15 K) = 161 . 755 g / mol 002 10.0 points Consider the following statements. Which are true? I) Vapor pressure is an equilibrium phe- nomenon. II) The smaller the IMF, the smaller the vapor pressure. III) The volume of a liquid does not affect the vapor pressure IV) Vapor pressure is temperature depen- dent. 1. I, III, and IV correct 2. I, II, and III 3. II, III, and IV 4. I, II, and IV 5. I, II, III, and IV 6. III, and IV 7. I, and III Explanation: As vapor particles encounter the surface of a solution and condense, their energy released makes another particle move as a vapor. This is an equilibrium process, and it is tempera- ture dependent. As IMF decreases, the va- por pressure increases, since only weak forces will be holding a substance together as a liq- uid. Vapor pressure also only occurs at the surface, so it does not matter how deep the liquid phase is. 003 10.0 points Which gas would you expect to have the largest value for the van der Waals constant “a”? 1. Ne 2. CH 4

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Version 317 – Exam 3 – mccord – (50970) 2 3. He 4. NH 3 correct Explanation: 004 10.0 points A 750.0 mL metal bulb is filled with 0.421 g of CH 4 and an unknown mass of NH 3 . If the total pressure in the bulb is 3.77 atm at 315 C, then how much NH 3 is also in the bulb? 1. 0.0323 grams 2. 2.39 grams 3. 14.5 grams 4. 1.17 grams 5. 0.549 grams correct 6. 0.996 grams 7. 0.00713 grams Explanation: n CH 4 = (0 . 421 g) · mol 16 g = 0 . 0263 mol V = 750 mL = 0 . 75 L T = 315 C + 273 = 588 K Applying the ideal gas law equation, P V = n R T n tot = P V R T = (3 . 77 atm) (0 . 75 L) (0 . 08206 L · atm mol · K ) (588 K) = 0 . 0585995 mol n tot = n NH 3 + n CH 4 n NH 3 = n tot - n CH 4 = 0 . 0585995 mol - 0 . 0263 mol = 0 . 0322995 mol m NH 3 = n NH e MM NH 3 = (0 . 0322995 mol) 17 g NH 3 1 mol NH 3 = 0 . 549091 g NH 3 005 10.0 points What is the root mean square speed of carbon dioxide molecules at 98 C? 1. 45.6 m · s 1 2. 574 m · s 1 3. 236 m · s 1 4. 459 m · s 1 correct 5. 153 m · s 1 Explanation: T = 98 C + 273 . 15 = 371 . 15 K MW = 44 . 0095 g / mol = 0 . 0440095 kg / mol R = 8 . 314 N K · mol The root mean square speed is v = radicalbigg 3 R T MW = radicalBigg 3 ( 8 . 314 N K · mol ) (371 . 15 K) 0 . 0440095 kg / mol = 458 . 635 m / s .
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