hemistry 122 (Tyvoll) ANSWERS T PRATIE EXAMINATIN I Fall 2005 1. Which statement is not correct? 1) A volatile liquid has a high boiling point. 2. Which of the following compounds is predicted to have the highest vapor pressure at STP? 2) 4 3. Which substance requires the smallest quantity of heat to melt a 1.0-g sample? 4) I 2 253.8 (MM, g mol- 1) 15.52 (Δfusion, kj/mol) Page Ref: Sec. 11.3 4. In which substance would the dispersion forces be greatest? 5) I2 5. Which would have the lowest boiling point? 1) l 2 6. Which of the following substances would exhibit dipole-dipole intermolecular forces? 5) N 3 7. Which idea is incorrect? 4) Polar molecules always have higher boiling points than nonpolar molecules. Page Ref: Sec. 11.6 8. Above the critical point for water (374 0 and 218 atmospheres), 3) water cannot be liquefied, no matter how much pressure is applied 9. What is the percent by mass of a solution made by dissolving 55.0 g Kl in 125 g water? 1) 30.6 10. What is the mole fraction of biphenyl, 12 10, in a solution prepared by dissolving 22.5 g 12 10 (s) in 285 g of benzene, 6 6 (l)? 2) 0.0384 Page Ref: Sec.12.2 11. What mass of phenol, 6 5, (FW = 94.11) must be dissolved in 25.0 g of naphthalene to produce a solution that is 0.22 m in phenol? 2) 0.52 g
12. alculate the molarity of a solution containing 40.0 g glucose ( 6 12 6, 180.0 g/mol) in enough liquid ethanol ( 2 5, 46.0 g/mol) to make 40.0 ml of solution. 4) 5.56 13. Which statement is incorrect? 1) When 40.0 ml of ethanol and 60.0 ml of water are mixed, the volume of the mixture is 100.0 ml. Page Ref: Sec. 12.3 14. Which of the following hypothetical steps is predicted to be exothermic? 3) separated solvent and solute molecules solution Page Ref: Sec. 12.3 15. Which statement is correct? 2) A saturated solution involves a dynamic equilibrium between dissolving and crystallization. Page Ref: Sec. 12.4
hemistry 122 (Tyvoll) Fall 2005 PRATIE EXAMINATIN I Part II Answers to Problems B 1. (5 points) Part II. ompletion and Short Answer. onsider the phase diagram shown at the right. Identify the solid, liquid and vapor phase regions on the actual diagram and then the processes represented by each of the following tie lines: Solid Liquid A sublimation temperature B melting point A boiling point Gas 2. (10 points) Melting Points and boiling points. (a) onsider the following elements and compounds: LiBr,, a, 3 Arrange these substances in order of increasing melting point. < 3 < LiBr < a Explain your reasoning as completely but as concisely as possible and 3 are molecular; LiBr and a are ionic; ionic compounds have stronger attractive forces than molecular compounds, so lowest melting points are for and 3. < 3 since has dispersion/dipole forces, while 3 has these plus -bonds. a > LiBr since in a, a is a 2+ and is 2-, while Li is Li 1+ and Br is Br 1-. Stronger forces of attraction when charge is larger. (b) ompounds A, B, and D (all of whose molecular structures have two carbon atoms) have the following boiling points: A, 141 0 ; B, -42.1 0 ;, 82.4 0 ; and D, 54.5 0. The structures of these four compounds are shown below; identify the compounds by placing the correct letter (A, B, or D) in the blank beneath the structure: 3 2 3 3 2 2 2 3 3 2 B D A Explain your reasoning as completely but as concisely as possible. B (dispersion); D (dispersion and dipole); (dispersion, dipole, -bonds); A (dispersion, dipole, more chances for -bonds);
3. (10 points) The molar heat of vaporization of methanol ( 3 ) is 38.0 kj/mol at 25 o. ow many joules (J) are required to convert exactly 1.000 kg of methanol (32.04 g/mol) from liquid to vapor? You must show all work for credit or partial credit!? J = (1.000 kg)(1000 g/kg)(1 mol/32.04 g)(38.0 kj/mol)(1000 J/kJ) = 11.19 x 10 6 J 4. (10 points) When 50.00 g of sorbitol, a molecular compound sometimes used as a sugar substitute, were dissolved in exactly 100.0 g of water, the boiling point of the water was raised from 100.000 0 to 101.406 0. If K b = 0.512 0 /molal for water, (a) calculate the molality of the solution (Show all work for credit!) ΔT = K b m (101.406 0 100.000 0 ) = (0.512 0 /molal) m = 1.406 0 m = (1.406 0 )(1 molal/0.512 0 ) = 2.746 m (b) calculate the molar mass of sorbitol mol sorbitol = (2.746 mol/1000 g 2 )(100.0 g 2 ) = 0.2746 mol sorbitol M = (50.00 g sorbitol/0.2746 mol sorbitol) = 182.1 g/mol 5. (10 points) ydrogen bonds can form between (a) water molecules and acetic acid molecules and between (b) water molecules and ethanol molecules. Draw all the water molecules that can hydrogen bond to an acetic acid molecule and all the water molecules that can hydrogen bond to an ethanol molecule (two pictures). acetic acid ethanol Based on your drawings, which should have the higher water solubility (affinity), acetic acid or ethanol? Acetic acid, since it has an extra oxygen site (=) for -bonding
6. (10 points) In a certain experiment, 144 mg of aspartame, an artificial sweetener, were dissolved in water and diluted to a final volume of exactly 25.00 ml. The osmotic pressure (Π), measured at 25 0, was 364 mmg. Use these data to calculate the molar mass (M) of aspartame. Π = MRT (364 mmg)(1 atm/760 mmg) = M (0.08205 L atm/mol K)(298 K) 0.479 atm = M (24.45 L/mol) M = 1.96 x 10-2 mol/l mol aspartame = (1.96 x 10-2 mol/l)(0.02500 L) = 4.90 x 10-4 mol aspartame M = (0.144 g aspartame)/( 4.90 x 10-4 mol aspartame) = 2.94 x 10 2 g/mol