hw 2 solution

hw 2 solution - reyes(mr29667 – H02 Colligative...

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Unformatted text preview: reyes (mr29667) – H02: Colligative Properties – Mccord – (90540) 1 This print-out should have 20 questions. Multiple-choice questions may continue on the next column or page – find 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 106 g toluene and 150 g benzene. 1. 0.626 2. 0.587 3. 0.704 4. 0.413 5. 0.597 6. 0.374 correct Explanation: m toluene = 106 g m benzene = 150 g n toulene = (106 g toluene) parenleftbigg 1 mol 92 . 14 g parenrightbigg = 1 . 15 mol toluene n benzene = (150 g benzene) parenleftbigg 1 mol 78 . 11 g parenrightbigg = 1 . 92 mol benzene The total number of moles of all species present is 1 . 15 mol + 1 . 92 mol = 3 . 07 mol The mole fraction of toluene is then X toluene = n toluene n total = 1 . 15 mol 3 . 07 mol = 0 . 374 002 10.0 points A colligative property of a solution is one that depends 1. only on the identity of the solute. 2. only on the concentration of the solu- tion. 3. only on the identity of the solvent. 4. on the identity of the solute and the con- centration of the solution. 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 different solubilities depending on the solvent. 003 10.0 points Calculate the vapor pressure at 20 ◦ C of a solution containing 1 . 209 g of naphthalene in 14 . 1 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: 144 . 463 torr. Explanation: m C 10 H 8 = 1 . 209 g m CHCl 3 = 14 . 1 g P CHCl 3 = 156 torr P CHCl 3 = X CHCl 3 P CHCl 3 = n CHCl 3 n CHCl 3 + n C 10 H 8 × P CHCl 3 = 14 . 1 g 119 . 38 g / mol 14 . 1 g 119 . 38 g / mol + 1 . 209 g 128 . 17 g / mol × 156 torr = 144 . 463 torr 004 10.0 points The vapor pressure of pure CH 2 Cl 2 (with molecular weight 85 g/mol) is 133 torr at ◦ 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? reyes (mr29667) – H02: Colligative Properties – Mccord – (90540) 2 1. 3.6 torr 2. 124 torr 3. 44 torr 4. 105 torr 5. 72 torr 6. 7.4 torr 7. 89 torr 8. 93 torr correct 9. 144 torr Explanation: For CH 2 Cl 2 , P = 133 torr MW = 85 g/mol For CH 2 Br 2 , P = 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 CH 2 Cl 2 + X CH 2 Br...
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hw 2 solution - reyes(mr29667 – H02 Colligative...

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