H02 Solutions - bleiweiss (emb853) H02: Colligative...

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bleiweiss (emb853) – H02: Colligative Properties – Mccord – (90540) 1 This print-out should have 20 questions. Multiple-choice questions may continue on the next column or page – fnd all choices beFore answering. Colligative properties From Chapter 17. 001 10.0 points Toluene (C 6 H 5 CH 3 ) is a liquid compound similar to benzene (C 6 H 6 ). Calculate the mole Fraction oF toluene in a solution that contains 76.9 g toluene and 266 g benzene. 1. 0.776 2. 0.289 3. 0.224 4. 0.803 5. 0.245 6. 0.197 correct Explanation: m toluene = 76.9 g m benzene = 266 g n toulene = (76 . 9 g toluene) ± 1 mol 92 . 14 g ² =0 . 835 mol toluene n benzene = (266 g benzene) ± 1 mol 78 . 11 g ² =3 . 41 mol benzene The total number oF moles oF all species present is 0 . 835 mol + 3 . 41 mol = 4 . 24 mol The mole Fraction oF toluene is then X toluene = n toluene n total = 0 . 835 mol 4 . 24 mol =0 . 197 002 10.0 points A colligative property oF a solution is one that depends 1. on the identity oF the solute and the con- centration oF the solution. 2. only on the identity oF the solvent. 3. only on the identity oF the solute. 4. only on the concentration oF the solu- tion. 5. on the identity oF the solvent and the concentration oF the solution. correct Explanation: A colligative property depends on the con- centration oF solution but NOT the identity oF the solute. It does depend on the identity oF solvent because substances have di±erent solubilities depending on the solvent. 003 10.0 points Calculate the vapor pressure at 20 C oF a solution containing 2 . 862 g oF naphthalene in 18 . 7 g oF chloroForm (CHCl 3 ). Naphthalene (C 10 H 8 ) has a low vapor pressure and may be assumed to be nonvolatile. The vapor pressure oF chloroForm at 20 C is 156 torr. Correct answer: 136 . 536 torr. Explanation: m C 10 H 8 =2 . 862 g m CHCl 3 = 18 . 7g P 0 CHCl 3 = 156 torr P CHCl 3 = X CHCl 3 P 0 CHCl 3 = n CHCl 3 n CHCl 3 + n C 10 H 8 × P 0 CHCl 3 = 18 . 7g 119 . 38 g / mol 18 . 7g 119 . 38 g / mol + 2 . 862 g 128 . 17 g / mol × 156 torr = 136 . 536 torr 004 10.0 points The vapor pressure oF pure CH 2 Cl 2 (with molecular weight 85 g/mol) is 133 torr at 0 C and the vapor pressure oF pure CH 2 Br 2 (with molecular weight 174 g/mol) is 11 torr at the same temperature. What is the total vapor pressure at 0 C oF a solution prepared From equal masses oF these two substances?
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bleiweiss (emb853) – H02: Colligative Properties – Mccord – (90540) 2 1. 124 torr 2. 144 torr 3. 72 torr 4. 93 torr correct 5. 105 torr 6. 44 torr 7. 89 torr 8. 3.6 torr 9. 7.4 torr Explanation: For CH 2 Cl 2 , P 0 = 133 torr MW = 85 g/mol For CH 2 Br 2 , P 0 = 11 torr MW = 174 g/mol The mole fractions are X CH 2 Cl 2 = n CH 2 Cl 2 n CH 2 Cl 2 + n CH 2 Cl 2 = g / 85 g / 85 + g / 174 =0 . 672 X CH 2 Br 2 = n CH 2 Br 2 n CH 2 Br 2 + n CH 2 Cl 2 = g / 174 g / 174 + g / 85 =0 . 328 Thus P solution = P CH 2 Cl 2 + P CH 2 Br 2 = X CH 2 Cl 2 P 0 CH 2 Cl 2 + X CH 2 Br 2 P 0 CH 2 Br 2 = (0 . 672)(133 torr) + (0 . 328)(11 torr)
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This note was uploaded on 09/01/2010 for the course CH 302 taught by Professor Holcombe during the Summer '07 term at University of Texas at Austin.

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H02 Solutions - bleiweiss (emb853) H02: Colligative...

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