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# exam2 - Version 169 – Exam 2 – McCord –(53580 1 This...

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Unformatted text preview: Version 169 – Exam 2 – McCord – (53580) 1 This print-out should have 27 questions. Multiple-choice questions may continue on the next column or page – find all choices before answering. McCord 1pm for 0 = ax 2 + bx + c x =- b ± √ b 2- 4 ac 2 a 001 10.0 points Consider the reaction 2 SO 2 (g) + O 2 (g) → 2 SO 3 (g) . At equilibrium at a certain temperature, the concentrations of SO 3 (g), SO 2 (g), and O 2 (g) are 0.12 M, 0.86 M, and 0.33 M, respectively. Calculate the value of K c for this reaction. 1. 0.42 2. 0.059 correct 3. 0.87 4. 1.31 5. 0.014 Explanation: 002 10.0 points A certain reaction has Δ H equal to 11.57 kJ/mol. This reaction is normally run at room temperature (25 ◦ C). At what new tem- perature should the reaction be run so that K is twice its value at 25 ◦ C? 1. 65 ◦ C 2. 94 ◦ C 3. 77 ◦ C correct 4. 50 ◦ C 5. 31 ◦ C 6. 135 ◦ C Explanation: T 1 = 25 ◦ C +273 = 298 K K 2 = 2 K 1 Δ H = 11570 J/mol ln K 2 K 1 = Δ H R parenleftbigg 1 T 1- 1 T 2 parenrightbigg R Δ H ln K 2 K 1 = 1 T 1- 1 T 2 T 2 = parenleftbigg 1 T 1- R Δ H ln K 2 K 1 parenrightbigg − 1 = parenleftbigg 1 298- 8 . 314 11570 ln2 parenrightbigg − 1 = 349 . 94 K = 77 ◦ C 003 10.0 points Pure liquids or solids do NOT appear in the equilibrium constant expression. 1. True correct 2. False Explanation: 004 10.0 points What is the pH of a solution containing 10 − 9 M HClO 4 ? 1. 6.996 correct 2. 9.000 3. 5.232 4. 5.000 5. 8.768 Explanation: 005 10.0 points Version 169 – Exam 2 – McCord – (53580) 2 What happens to the concentration of HI(g) when the total pressure on the equi- librium reaction 2 HCl(g) + I 2 (s) ⇀ ↽ 2 HI(g) + Cl 2 (g) is increased (by compression)? 1. decreases correct 2. remains the same 3. Unable to determine 4. increases Explanation: Increasing the total pressure on the sys- tem by decreasing its volume will shift the equilibrium toward the side of the reaction with fewer numbers of moles of gaseous com- ponents. If the total number of moles of gas is the same on the product and reactant sides of the balanced chemical equation, then changing the pressure will have little or no ef- fect on the equilibrium distribution of species present. 006 10.0 points If 75.0 mL of a 0.026 M HCl solution is mixed with 25.0 mL of a 0.15 M NaOH solution, what is the pH of the final mixture? 1. 2.74 2. 1.74 3. 4.15 4. 3.13 5. 10.37 6. 13.09 7. 12.26 correct 8. 7.38 Explanation: [HCl] = 0.026 M V HCl = 75.0 mL [NaOH] = 0.15 M V NaOH = 25.0 mL To determine the pH of the final mixture, we need to determine how much H or OH is left over after the reaction. Remember that for complete neutralization we need H and OH in equal molar amounts: H + + OH − → H 2 O First calculate how many moles of H + and OH − we have: ? mol H + = 0 . 075 L × . 026 mol HCl 1 L × 1 mol H + 1 mol HCl = 1 . 95 × 10 − 3 mol H + ? mol OH − = 0 . 025 L × . 15 mol NaOH 1 L × 1 mol OH − 1 mol NaOH = 3 . 75 × 10 − 3 mol OH − Now we can see that we have more OH...
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exam2 - Version 169 – Exam 2 – McCord –(53580 1 This...

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