NAME ______________________________________________ © Keeney-Kennicutt, 2010 B1 CHEMISTRY 102 FALL 2010 EXAM 3 FORM B SECTION 501 DR. KEENEY-KENNICUTT Directions: (1) Put your name on PART 1 and your name and signature on PART 2 of the exam where indicated. (2) Sign the Aggie Code on PART 2 of this exam. (3) Each multiple choice question is actually 2 questions on your scanning sheet. If you are sure of an answer, put the same answer down for both questions for 5 pts. If you cannot decide between two answers, put your best answer down for the first (odd) question and the other answer down for the second (even) question. If you get the first one correct you'll get 3 pts; if you get the second one correct you’ll get 2 pts. If there is an ambiguous multiple choice question, use the last page to explain your answer. (4) Do NOT write on the envelope. (5) When finished, put everything in the envelope and wait to be excused. At the table, take everything out of the envelope. You can pick up the multiple choice part with the answers outside my office after 3pm. (6) There are a total of 36 questions (20 actual questions). PART 1 1&2. The conjugate base of HCO3– is: (a) H2CO3 (b) OH– (c) H2O (d) CO32– (e) none of these 3&4. The compound, HBrO is a:________________ . (a) weak acid (b) strong acid (c) weak base (d) strong base (e) neutral 5&6. Which equilibrium has the thermodynamic equilibrium constant expression: Kthermo = (px)/[Y]3 (a) Y(aq) →← 3X(g) (b) 3Y(s) →← X(g) (c) 3Y(aq) →← 3Z(s) + X(g) (d) X(g) →← Z(l) + 3Y(aq) (e) X(g) →← 3Y(g) + 3Z(s) 7&8. Consider the titrations of the pairs of aqueous acids and bases listed at the left. For which pair is the pH at the equivalence point stated INCORRECTLY? Acid-Base Pair pH at Equivalence Point (a) HClO3 + KOH equal to 7 (b) HBr + NH3 less than 7 (c) CH3COOH + NaOH more than 7 (d) HNO3 + Ba(OH)2 equal to 7© Keeney-Kennicutt, 2010 B2 (e) HF + NaOH less than 7 9&10.The hydrolysis constant used when finding the pH of a solution of NaCN is: (a) KwKa for Na+ (b) Ka for Na+Kw (c) Ka for HCNKw (d) KwKa for HCN (e) Ka for HCNKb for CN− 11&12. Which will be a buffer when equal volumes of the following solutions are mixed? (a) 0.1 M HCN and 0.2 M NaCN (b) 0.1 M HClO3 and 0.2 M NaClO3 (c) 0.1 M NH3 and 0.1 M NaCl (d) 0.1 M CH3NH2 and 0.2 M NH4Cl (e) 0.1 M NaOH and 0.1 M NaF 13&14. Consider 0.1 M solutions of the following weak acids: HClO Ka = 3.5 x 10−8 HNO2 Ka = 4.5 x 10−4 Which of the following statements is NOT correct? (a) Nitrous acid is a stronger acid than hypochlorous acid. (b) [ClO−] in HClO solution > [NO2−] in HNO2 solution. (c) The concentration of OH− ions is greater in the HClO solution. (d) The pH of the HNO2 solution is lower than the pH of the HClO solution. (e) [H+] in HNO2 solution > [H+] in HClO solution. 15&16. In a sample of pure water, only one of the following statements is ALWAYS true at all conditions of temperature and pressure. Which one is ALWAYS true? (a) [H3O+] x [OH−] = 1.0 x 10−14 (b) pH = 7.0 (c) [H3O+] = [OH−] (d) pOH + pH = 14.0 (e) [H3O+] = 1.0 x 10−7 M© Keeney-Kennicutt, 2010 B3 17&18. Calculate the thermodynamic equilibrium constant at 25oC for a reaction for which ∆Go = +23.0 kJ per mol rxn? (a) 9.14 (b) 9.3 x 10–5 (c) 1.0 x 104 (d) 4.8 x 10–10 (e) 0.0034 19&20. What is the pH of a 4.00 x 10–2 M NaOH(aq) solution? (a) 12.60 (b) 10.76 (c) 11.22 (d) 13.00 (e) 12.41 21&22. What is the percent ionization of a 0.40 M solution of (CH3)3N? (a) 3.0% (b) 0.73% (c) 2.2% (d) 1.7% (e) 1.4%© Keeney-Kennicutt, 2010 B4 23&24. Calculate the pH of the solution resulting from the addition of 30.0 mL of 0.300 M HBr to 70.0 mL of 0.100 M NaOH. (a) 7.00 (b) 2.61 (c) 1.70 (d) 2.30 (e) 12.85 25&26. What is the pH of a 0.20 M NH4Cl solution? (a) 7.00 (b) 4.17 (c) 9.91 (d) 9.12 (e) 4.98© Keeney-Kennicutt, 2010 B5 27&28. Calculate the pH that results when 30.0 mL of 1.00 M NaOH is added to 500.0 mL of a solution composed of 0.200 M dimethylamine and 0.100 M dimethylammonium chloride. (a) 10.98 (b) 10.84 (c) 10.01 (d) 11.68 (e) 2.35 The following 5 questions (29 – 35) deal with a single titration: 29&30. A 40.0 mL sample of 0.300 M formic acid is titrated with 0.200 M NaOH. Calculate the initial pH before the titration is begun. (a) 1.93 (b) 2.13 (c) 2.41 (d) 2.85 (e) 0.55 OVER ⇒© Keeney-Kennicutt, 2010 B6 31&32. A 40.0 mL sample of 0.300 M formic acid is titrated with 0.200 M NaOH. Calculate the pH after 25.0 mL of 0.200 M NaOH has been added. (a) 3.11 (b) 3.25 (c) 3.60 (d) 3.91 (e) 3.47© Keeney-Kennicutt, 2010 B7 CHEMISTRY 102 EXAM 3 Form B FALL 2010 Section 501 NAME (Please blockprint) PART 2 Please read and sign: “On my honor, as an Aggie, I have neither given nor received unauthorized aid on this exam.” _______________________________________________ (5 pts) 33. A 40.0 mL sample of 0.300 M formic acid is titrated with 0.200 M NaOH. Calculate the pH at the equivalence point. (5 pts) 34. A 40.0 mL sample of 0.300 M formic acid is titrated with 0.200 M NaOH. Calculate the pH after 70.0 mL of 0.200 M NaOH is added. OVER ⇒© Keeney-Kennicutt, 2010 B8 (5 pts) 35. A 40.0 mL sample of 0.300 M formic acid is titrated with 0.200 M NaOH. Using the answers to Questions 29-34, sketch the titration curve with pH on the vertical axis and milliliters of base added on the horizontal axis. Label the axes and plot your 4 points. Point out the buffer region and the equivalence point. If you cannot complete the calculations, sketch what the curve should look like for partial credit. 36. A weak acid indicator, HIn (Ka = 1.0 x 10–3) ionizes as follows: HIn →← H+ + In– clear red (2 pts) (a) The endpoint of an acid-base titration that uses this indicator will be at pH = ______________. (2 pts) (b) Explain your answer to (a) using the equilibrium expression. (1 pt) (c) The color at pH = 5 will be __________________.© Keeney-Kennicutt, 2010 B9 SCRAP PAPER OR COMMENTS ON EXAM