Name____________________ Page 1 of 8 Chemistry 1A, Fall 2007 KEY Midterm Exam #2 October 16, 2007 (90 min, closed book) Name:__________________________________ SID:___________________________________ GSI Name:________________ • The test consists of 6 short answer questions and a page of multiple choice questions. • Put your written answers in the boxes provided. Answers outside the boxes may not be considered in grading. • Show your work to receive the maximum credit possible. • Write your name on every page of the exam. Question Page Points Score Question 1 2 10 Question 2 3 6 Question 3 3 8 Question 4abc 4 12 Question 4def 5 10 Question 5 5 8 Question 6 7 14 Multiple Choice 8 12 Total 80 Useful Equations and Constants: pH = - log[H3O+] pX = - log X ][][logHAApKpHa−+= Strong acids and bases: HCl LiOH HNO3 NaOH H2SO4 KOH HClO4 HBr HIName____________________ Page 2 of 8 Acid-base chemistry (10 points) 1. A group of students measures and records the pH of each of the 0.10 M aqueous acid solutions shown below: a) List the four acids in order of increasing dissociation. D < C < B < A b) Based on this information, Marco states that "The solution of pH 1.1 will require the most base to titrate it to the equivalence point since it is the strongest acid and its pH is the farthest away from 7." Jovan disagrees: "I think that all four solutions will require the same amount of base to reach the equivalence point because they all contain the same concentration of acid." Katrina does not agree with either of these statements: "Since we don't know the identity of the acids, we can't determine which one will require the most base to titrate it to the equivalence point." Which student do you agree with? I agree with Marco. I agree with Jovan. I agree with Katrina. I don't agree with any of them. c) Explain your reasoning. Several logical explanations are acceptable. • I agree with Jovan- assume same volume, given same concentration, each flask contains the same number of moles of acid so will require the same number of moles of base to titrate • I agree with Katrina- we are not told whether or not all the acids are monoprotic so they might take different amounts of base to titrate to equivalence, a diprotic acid will take twice as much base to titrate it fully • I don't agree with any of them.- we can’t assume the solutions of acid all have the same volume so although the concentrations are equal, we don’t know the moles of acid in each flask A pH = 1.1 B pH = 2.4 C pH = 2.9 D pH = 5.1Name____________________ Page 3 of 8 Two-phase equilibria (6 points) 2. Suppose that you establish an equilibrium between NH3 (g) and NH3 (aq) in a closed 2.0 L container at 25°C. NH3 (g) NH3 (aq) For each of the following, circle the correct answer: a) If the pressure of NH3 (g) is increased, the molar concentration of NH3 (aq) will: increase stay the same decrease b) If the temperature is increased from 25°C to 80°C, the molar concentration of NH3 (aq) will: increase stay the same decrease c) If the total pressure is increased by adding air, the molar concentration of NH3 (aq) will: increase stay the same decrease Solubility equilibria (8 points) 3. Consider the solubility of Cu(OH)2. Cu(OH)2 (s) ' Cu2+ (aq) + 2 OH- (aq) a) The pH of a solution of 1 mole Cu(OH)2 (s) dissolved in 1.0 L of water is 7.66. Does all the Cu(OH)2 dissolve? yes no not enough information b) Determine the solubility of Cu(OH)2 in 1.0 M NaOH (aq). The Ksp is 4.8×10-20. Cu(OH)2 (s) ' Cu2+ (aq) + 2OH- (aq) equilibrium S 1M Ksp = 4.8×10-20 = [Cu2+ ] [OH-]2 = [Cu2+ ] [1M-]2 [Cu2+ ] = 4.8×10-20 which is the solubility of the saltName____________________ Page 4 of 8 Acid-Base Equilibria (12 points) 4. NO2 gas can dissolve in water and react to make acids. This pollutant is one of the causes of acid rain 2NO2 (g) + H2O (l) HNO3 (aq) + HNO2 (aq) nitric acid nitrous acid ONOOH ONOH a) Write the chemical reaction for the reaction of nitric acid with water. Circle the conjugate base of HNO3. HNO3(aq) + H2O(l) H3O+(aq) + NO3-(aq) b) Write the chemical reaction for the reaction of nitrous acid with water. Circle the conjugate base of HNO2. HNO2(aq) + H2O(l) H3O+(aq) + NO2-(aq) c) Based on the relative stabilities of the conjugate bases produced, which acid is stronger? Circle one and explain your reasoning. nitric acid nitrous acid NO3- is more stable than NO2- because it has more oxygens. Oxygen is electronegative and will stabilize the extra e- in the ion. Also, NO3- has 3 resonance structures while NO2- has only two. Resonance allows the electron density to be spread over the entire molecule further lowering the energy of the ion. ONO-1ONO-1ONO-1OOO ONO-1ONO-1Name____________________ Page 5 of 8 #4) Continued (10 points) d) A buffer solution is made by mixing HNO2 and NaNO2. What is the pH of a 1.00 L buffer solution that is 0.100 M HNO2 and 0.150 M NaNO2? (Ka = 4.3 × 10-4) pKa = -log Ka pKa = 3.37 pH = pKa + log -[][]AHA pH = 3.37 + log (0.150NO2-/0.100HNO2) = 3.55 e) Consider adding a strong base to a buffer. Write the chemical reaction that occurs when the strong base NaOH reacts with the buffer solution from part 4d. HNO2 (aq) + NaOH (aq) → NaNO2 (aq) + H2O (l) f) If you add 20.00 mL of 1.00 M NaOH to the solution, estimate the resulting pH. (circle one) 1 2 3 4 5 6 7 8 9Name____________________ Page 6 of 8 Equilibrium (8 points) 5. Cyclopropane can convert to propene. Given the reaction and data below, answer the following questions. Assume that pressure and temperature are constant. CCCCCCHHHHHHHHHHHH 0.000.020.040.060.080.100.120 5 10 15 20Time (sec)Concentration (M)[cyclopropane][propene] a) Which is true for the reaction at 2 seconds? (circle one) Q < K Q = K Q > K b) Which is true for the reaction at 17 seconds? (circle one) Q < K Q = K Q > K c) What is the value of the equilibrium constant for the reaction? fig) sig 1 with 2just (or