Exam2 Answer Key

Chemistry 123 Exam 2 October 18, 2005 KEY Answer questions 1 - 3 about the following reaction 2 Mg (s) + O2 (g) → 2 MgO (s) ΔH = –1204 kJ 1. This reaction is (a) exothermic (b) endothermic 2. How many grams of MgO are produced during an enthalpy change of –96.0 kJ? (a) 80.6 g (b) 6.43 g (c) 3.21 g (d) cannot determine 3. What is the enthalpy change when 7.50 g of MgO (s) decomposes to Mg (s) and O2 (g)? (a) –223 kJ (b) +223 kJ (c) –112 kJ (d) +112 kJ (e) none of these 4. In Lab #5 you studied the following reaction: 2 HCl (aq) + Na2CO3 (s)  2 NaCl (aq) + CO2 (g) + H2O (l) How many grams of sodium carbonate must have reacted if 75.0 mL of 0.25 M HCl produced CO2 with a pressure of 1.5 atm in a volume of 150 mL at 25ºC? (a) 1.99 g (b) 1.9 g (c) 0.995 g (d) 0.99 g (e) none of these I’ll accept either answer 5. Hydrogen gas is produced when zinc reacts with sulfuric acid: Zn (s) + H2SO4 (aq) → ZnSO4 (aq) + H2 (g) If 159 mL of wet H2 is collected over water at 24°C and a barometric pressure of 739 torr, how many grams of Zn have been consumed? (a) 0.402 g (b) 0.415 g (c) 0.0124 g (d) 0.128 g (e) none of these 6. At 25°C and constant pressure, carbon monoxide gas combines with oxygen gas to give carbon dioxide gas with the evolution of 10.1 kJ per gram of carbon monoxide consumed. What is the value of ΔH for the balanced reaction? (a) –566 kJ/mol (b) –283 kJ/mol (c) 283 kJ/mol (d) 141 kJ/mol (e) –141 kJ/mol 7. Titration of 25.00 mL of sulfuric acid requires 17.66 mL of 0.358 M sodium hydroxide. What is the molarity of the sulfuric acid? (a) 0.126 M (b) 0.253 M (c) 0.506 M (d) 00741 M (e) none of these Answer questions 8 - 10 from the following information: A quantity of N2 gas originally held at 3.80 atm pressure in a 1.00-L container at 26°C is transferred to a 10.0-L container at 20°C. A quantity of O2 gas originally at 4.75 atm and 26°C in a 5.00-L container is transferred to this same container. 8. What is the total pressure in the new container? (a) 0.372 atm (b) 2.327 atm (c) 2.70 atm (d) 4.28 atm (e) none of these Chemistry 123 Exam 2 Harwood/Fall05 9. What is the partial pressure of O2 gas in the new container? (a) 0.372 atm (b) 2.33 atm (c) 2.38 atm (d) none of these 10. What is the mole fraction of N2 gas in the new container? (a) 0.138 (b) 0.155 (c) 0.372 (d) none of these 11. Consider the following data: Metal Mass (g) Specific Heat (J/g ˚C) Temperature (˚C) Al Cu 10 0.900 40 30 0.385 60 When these two metals are placed in contact, which of the following will take place? (a) Heat will flow from Al to Cu because Cu has a larger mass. (b) Heat will flow from Cu to Al because Cu has a larger mass. (c) Heat will flow from Cu to Al because Cu has a larger heat capacity. (d) Heat will flow from Cu to Al because Cu is at a higher temperature. (e) No heat will flow in either direction. 12. Consider the following reactions. ΔH° for reaction 3 is IF (g) + F2 (g) → IF3 (g) ΔH° = –390. kJ (1) IF (g) + 2 F2 (g) → IF5 (g) ΔH° = –745 kJ (2) IF5 (g) → IF3 (g) + F2 (g) (a) +355 kJ (3) (b) –1135 kJ (c) +1135 kJ (d) +35 kJ (e) –35 kJ 13. How many grams of water can be heated from 20.0°C to 25.0°C by the addition of 324 J of energy? (a) 15 (b) 21 (c) 1.4 × 10 3 (d) 6.8 × 10 3 (e) 390 Consider these changes for questions 14 & 15: I. Hg(l) → Hg(g) II. 3 O2(g) → 2 O3(g) III. CuSO4 • 5 H2O(s) → CuSO4(s) + 5 H2O(g) IV. H2(g) + F2(g) → 2 HF(g) 14. At constant pressure, in which of the reactions is work done by the system on the surroundings? (a) I only (b) II only (c) III only 2 (d) I and II (e) I and III Chemistry 123 Exam 2 Harwood/Fall05 15. In which of them is no work done? (a) I only (b) II only (c) III only (d) IV only (e) I and III 16. The specific heat of bromine liquid is 0.226 J/g K. What is the molar heat capacity (in J/mol K) of bromine liquid? (a) 707 (b) 36.1 (c) 18.1 (d) 9.05 (e) 0.226 17. How much heat is required to raise the temperature of 50.0 g of ice at -10.0ºC to 40.0ºC? (Specific heat of water is 4.184 J/g ºC; specific heat of ice is 2.092 J/g ºC; ΔHfusion (H2O, 0ºC) = 6.008 kJ/mol; ΔHvaporization (H2O, 100ºC) = 40.67 kJ/mol) (a) 10.5 kJ (b) 9.41 kJ (c) 16.7 kJ (d) 26.1 kJ (e) none of these 18. Arrange the following gases in order of decreasing average molecular speed at 25ºC. argon, fluorine, carbon dioxide, oxygen, helium (a) helium > oxygen > fluorine > carbon dioxide > argon (b) helium > oxygen > fluorine > argon > carbon dioxide (c) argon > oxygen > helium > carbon dioxide > fluorine (d) carbon dioxide > argon > fluorine > oxygen > helium (e) carbon dioxide > fluorine > argon > oxygen > helium 19. Suppose that in a known volume of He, there are 6.022 × 1023 molecules and 4.00 g at 1.00 atm and 0ºC. How many molecules and how many grams of Cl2 are there under the same conditions? (a) 6.022 × 1023 molecules and 35.45 g (b) 1.00 molecules and 4.00 g (c) 6.022 × 1023 molecules and 70.90 g (d) 2(6.022 × 1023) molecules and 35.45 g (e) 8(6.022 × 1023) molecules and 70.90 g 20. A mixture of H2 (g) and Cl2 (g) exerts a total pressure of 2.50 atm. If the mole fraction of H2 is 0.490, what is the partial pressure (in atm) of H2? (a) 0.253 (b) 0.396 (c) 0.606 3 (d) 1.22 (e) 2.50