The more powerful and ex- tensive the intermolecular forces that exist between molecules, the higher the expected boiling point will be. Since NaCl is an ionic compound, you would expect that it would have the high- est boiling point, as it has ion-ion interactions as its primary intermolecular force. Ar only has the weakest intermolecular force, dispersion forces, so it would have the lowest boiling point. Both NH 3 and H 2 O have hydrogen bond- ing; however, two hydrogen bonds can be made from each H 2 O molecule (each H 2 O has two hydrogen atoms and two lone pairs on oxygen) but only one hydrogen bond can be made from each NH 3 molecule (three hydro- gen atoms, but only one lone pair on nitro- gen). So hydrogen bonding would be more ex- tensive in H 2 O, and it would exhibit a higher boiling point than NH 3 . 028 (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. greater than 3. the same as 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 -

Panjwani, Sameer – Exam 1 – Due: Sep 18 2007, 3:00 pm – Inst: James Holcombe 10 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. 029 (part 1 of 1) 10 points 25.0 g of ethanol (C 2 H 5 OH, of MW 46 g and density 0.789 g/mL) is added to 40.0 g of water (of MW 18 g and density 1.0 g/mL) to yield a solution that is 68.4 mL in volume and has a density of 0.950 g/mL. What is the molality of ethanol in this solution? 1. 7.95 m 2. 0.516 m 3. 8.36 m 4. 10.7 m 5. 13.6 m correct Explanation: 030 (part 1 of 1) 10 points Each of the following samples was placed in 1 liter of water. I) 0.6 mol NaOH II) 0.7 mol KCl III) 0.5 mol Na 2 NO 3 IV) 1 mol of sugar Rank the solutions that are made in terms of increasing order of boiling point elevation. (Remember your solubility rules.) 1. IV, I, II, III correct 2. II, IV, I, III 3. IV, II, I, III 4. III, I, II, IV 5. III, I, IV, II Explanation: Colligative properties of a solution depend on the number of solute particles in solution, not the type. I) will have 1.2 mol of particles because NaOH completely dissociates in water. II) will have 1.4 mol of particles because KCl completely dissociates in water. III) will have 1.5 mol of particles because Na 2 CO 3 completely dissociates in water. IV) will have 1 mol of sugar in solution because it is soluble but does not dissociate. The higher the concentration, the greater the boiling point elevation, so IV > I > II > III . 031 (part 1 of 1) 10 points Consider two liquids A and B. The vapor pressure of pure A (molecular weight = 50 g/mol) is 225 torr at 25 ◦ C and the vapor pressure of pure B (molecular weight = 75 g/mol) is 90 torr at the same temperature.