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Unformatted text preview: Version 131 – Exam 1 – Mccord – (52445) 1 This printout should have 28 questions. Multiplechoice questions may continue on the next column or page – find all choices before answering. 001 10.0 points Substance A has undergone a phase tran sition (under constant pressure) where ΔH = 10kJ / mol and ΔS = 36J / K · mol. What phase transition could have occurred? 1. condensation 2. deposition 3. freezing 4. melting correct Explanation: Melting is the only phase transition listed where both ΔH and ΔS are positive values. Condensation (gas to liquid), freezing (liqid to solid) and deposition (gas to solid) all have a negative ΔH and ΔS. 002 10.0 points K f = 1 . 86 ◦ C/ m for water. The freezing point of a 0.01 molal so lution of the weak monoprotic acid HA is . 0206 ◦ C. What is the percent ionization in this aqueous solution? 1. 4 2. 11 correct 3. 18 4. 8 5. 22 Explanation: HA ⇀ ↽ H + + A − ini, m . 01 Δ, m x x x fin, m . 01 x x x Total molality = 0 . 01 x + x + x = 0 . 01 + x Δ T f = T f T f = 0 . 00 ◦ C ( . 0206 ◦ C) = 0 . 0206 ◦ C Δ T f = k f · m m = Δ T f k f = . 0206 ◦ C 1 . 86 ◦ C /m = 0 . 011075 m . 01 + x = 0 . 011075 x = 0 . 001075 % ionization = . 0010175 × 100% . 01 = 11% 003 10.0 points How many grams of water at 36.0 ◦ C must be added to 10.0 grams of ice at 20 ◦ C to result in ONLY liquid water at 0 ◦ C? 1. 11.2 g 2. 68.8 g 3. 12.5 g 4. 56.6 g 5. 24.9 g correct 6. 22.4 g 7. 74.0 g 8. 109.2 g 9. 1.25 g 10. 21.2 g Explanation: The temperature of the 10.0 g ice must be raised from 20 ◦ C to 0 ◦ C: q heat ice = m c Δ T = (10 . 0 g) parenleftbigg 2 . 09 J g · K parenrightbigg (20 ◦ C) = 418 J and then melted: Version 131 – Exam 1 – Mccord – (52445) 2 q melt ice = m c = (10 . 0 g)(334 J / g) = 3340 J The total energy needed is 3758 J; this en ergy comes from the the original water which must cool from 36.0 ◦ C to 0 ◦ C: q total = m c Δ T m = q c Δ T = 3758 J parenleftBig 4 . 184 J g · ◦ C parenrightBig (36 ◦ C) = 24 . 9495 g 004 10.0 points Glycerol (a nonelectrolyte that can be used as antifreeze in a car radiator) has a density of 1.261 g/mL and a molecular weight of 92 g. If 100 mL are mixed with 1 kg of water, what is a good estimate of the temperature at which this solution will freeze ? K f water = 1.861 ◦ C/molality. 1. 0.0 ◦ C 2. 12.4 ◦ C 3. 0.23 ◦ C 4. 2.6 ◦ C correct 5. 4.8 ◦ C Explanation: 005 10.0 points A solution of 0.90 g of an unknown non electrolyte in 300 mL of water at 27 ◦ C has an osmotic pressure of 38.4 torr. What is the molecular weight of this compound? (Ne glect volume change due to solute, R = 0.0821 L · atm/mol · K) 1. 3 . 78 × 10 2 g/mol 2. 4.72 x10 5 g/mol 3. 1 . 52 × 10 3 g/mol 4. 5.81 x10 4 g/mol 5. 1 . 46 × 10 3 g/mol correct Explanation: T = 27 ◦ C + 273 = 300 K π = 38 . 4 760 atm π = M RT , where M is molar concentra tion....
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 Spring '07
 Holcombe
 pH, Vapor pressure, Phase transition

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