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CH 302 HW2

CH 302 HW2 - Thompson Margaret Homework 2 Due 11:00 am Inst...

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Thompson, Margaret – Homework 2 – Due: Sep 18 2007, 11:00 am – Inst: James Holcombe 1 This print-out should have 25 questions. Multiple-choice questions may continue on the next column or page – find all choices before answering. The due time is Central time. 001 (part 1 of 1) 10 points The temperature at which water boils can be increased by adding some solute, like ethylene glycol. This is known as a colligative property. 1. False 2. True correct Explanation: Colligative properties depend on the num- ber of particles dissolved in the solution, not on the properties of the components. This colligative property is the boiling point elevation of water. 002 (part 1 of 1) 10 points The colligative effects of 1 molal sugar solu- tion would be ? 1 molal sodium chloride solution. 1. less than correct 2. the same as 3. greater than Explanation: Sugar is a non-electrolyte so the concen- tration of particles is the concentration of dissolved sugar. Soduim chloride is an electrolyte which dis- sociates in solution, giving twice as many par- ticles as dissolved: NaCl -→ Na + + Cl - Since colligative properties depend on the number of particles in solution, the colligative effect doubles for the NaCl(aq), compared to any identical concentration of sugar. 003 (part 1 of 1) 10 points The vapor pressure of a volatile component in a solution decreases as its mole fraction decreases. This is known as 1. LeChatelier’s Principle. 2. Bragg’s Law. 3. Raoult’s Law. correct 4. Henry’s Law. Explanation: According to Raoult’s Law, the vapor pres- sure of the volatile component of a solution equals its mole fraction in the solution times the vapor pressure of the pure solvent. P = X P 0 004 (part 1 of 3) 10 points Consider the following pressure diagram for a binary liquid containing A and B. 0 1 x A 90 torr 30 torr If 800 g of A (100 g / mol) and 1140 g of B (95 g / mol) are mixed, what is the vapor pressure of B? 1. 12 torr 2. 90 torr 3. 65 torr 4. 53 torr correct 5. 30 torr Explanation: We have 800 g 100 g / mol = 8 mol A and 1140 g 95 g / mol = 12 mol B, so the mole fraction

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Thompson, Margaret – Homework 2 – Due: Sep 18 2007, 11:00 am – Inst: James Holcombe 2 of A is x A = mol A total mol = 8 mol 8 mol + 12 mol = 0 . 4 0 1 0 . 4 30 torr 53 torr 005 (part 2 of 3) 10 points What is the corresponding vapor pressure of A? 1. 90 torr 2. 30 torr 3. 65 torr 4. 12 torr correct 5. 53 torr Explanation: 0 1 0 . 4 30 torr 12 torr 006 (part 3 of 3) 10 points What is the corresponding total vapor pres- sure? 1. 53 torr 2. 12 torr 3. 30 torr 4. 65 torr correct 5. 90 torr Explanation: 0 1 0 . 4 30 torr 65 torr 007 (part 1 of 1) 10 points 1,1-Dichloroethane (CH 3 CHCl 2 ) has a va- por pressure of 228 Torr at 25 C; at the same temperature, 1,1-dichlorotetrafluoroethane (CF 3 CCl 2 F) has a vapor pressure of 79 Torr. What mass of 1,1-dichloroethane must be mixed with 280 g of 1,1-dichlorotetrafluoro- ethane to give give a solution with vapor pressure 157 Torr at 25 C? Assume ideal behavior.
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