Northern Arizona University Exam #3. Section 2, Spring 2006 April 21, 2006

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1 Northern Arizona University Exam #3 CHM 152, General Chemistry II Dr. Brandon Cruickshank Section 2, Spring 2006 April 21, 2006 Name ID # INSTRUCTIONS: Code the answers to the True-False and Multiple-Choice questions on the scantron form. Mark A for true and B for false. There is only one correct answer for each multiple-choice question. There is no partial credit given for this section. Show all work on the problems section because partial credit is awarded for this section. Below your ID# above, answer the following question. If you could be a professional athlete, what sport would you choose? You will receive 1 bonus point. There are 90 points on this exam. GOOD LUCK! ENJOY!! PART I: True-false statements (3 points each) 1. You prepare an CH 3 COOH/NaCH 3 COO buffer solution with a ph of The solution will contain a greater concentration of CH 3 COOH than NaCH 3 COO. [K a (CH 3 COOH) = ] F 2. F B F fits the definition of a Lewis acid, but not that of a Brønsted acid. T F 3. If K > 1, then ΔG is positive. F 4. A 0.10 M NaH 2 PO 4 and 0.10 M Na 2 HPO 4 solution would function as a buffer solution. T PART II: Multiple Choice (3 points each) 5. Acid strength decreases in the series: HSO 4 > CH3 COOH > H 2 CO 3 > HCN. Which of the following is the weakest base? [a] HCO 3 [b] CN [c] CH 3 COO 2 [d] SO 4 [e] All will have the same strength. 6. Which of the following salts will produce a neutral solution when dissolved in water? [a] LiBr [b] NaF [c] NH 4 NO 3 [d] KOH [e] NaCN 7. K a for HX is What is the ph of a 0.15 M solution of NaX? [a] 2.85 [b] 4.97 [c] 7.87 [d] 9.03 [e] 11.15

2 Exam 3 Spring 2006 CHM 152, Sec 2 Dr. Cruickshank 8. Which of the following statements are true? I. HF is a stronger acid than HI. II. HClO 3 is a stronger acid than HClO. III. H 3 PO 4 is a stronger acid than H 2 SO 4. IV. HNO 2 is a weak acid. [a] I and IV [b] II and IV [c] II, III, and IV [d] I and III [e] II only 9. In the reaction shown below, NH 3 is acting as a(n) base. I. Arrhenius II. Brønsted-Lowry III. Lewis.. + N H3 + H2O NH4 + OH [a] I and II [b] II and III [c] I and III [d] II only [e] I, II, and III 10. Which of the following buffers would be the best choice for preparing a ph = 4.5 buffer? [a] HCN/CN, [K a (HCN) = ] [b] HOCl/OCl, [K a (HOCl) = ] [c] H 2 CO 3 /HCO 3 [Ka (H 2 CO 3 ) = ] [d] C 6 H 5 NH 2 /C 6 H 5 NH 3 [Kb (C 6 H 5 NH 2 = ] [e] none of these could be used to prepare a ph = 4.5 buffer 11. Consider a solution prepared by dissolving 0.35 mol of solid CH 3 NH 3 + Cl in 1.00 L of 1.1 M CH3 NH 2 (methylamine). The K b for methylamine is If 0.01 mole of KOH is added to this buffer solution, the ph of the solution will slightly because the KOH reacts with present in solution. + [a] decrease, CH 3 NH 2 [b] increase, CH 3 NH 2 [c] decrease, CH 3 NH 3 + [d] increase, CH 3 NH 3 [e] This is a buffer solution, so the ph does not change. 12. Calculate the ph of a buffer solution prepared by dissolving 0.80 mole of cyanic acid (HCNO) and 0.20 mole of sodium cyanate (NaCNO) in enough water to make 2.0 L of solution. [K a (HCNO) = ] [a] 2.85 [b] 3.10 [c] 3.70 [d] 4.30 [e] Calculate the ph of the buffer in Problem 12 after 0.10 mole of KOH is added to the buffer. Assume no change in volume upon addition of the KOH. [a] 2.75 [b] 3.22 [c] 3.33 [d] 4.07 [e] What ratio of [CH 3 COOH]/[CH 3 COO ] is required to prepare a acetic acid/ sodium acetate buffer solution with a ph of 4.20? [K a (CH 3 COOH) = ] [a] 1.7 [b] 0.29 [c] 3.5 [d] 0.58 [e] none of these 15. How many milliliters of 1.00 M HCl will be needed to completely react with 5.00 ml of 2.00 M Ba(OH) 2? [a] 10.0 ml [b] 20.0 ml [c] 5.00 ml [d] 1.00 ml [e] none of these (2)

3 Spring 2006 Exam 3 Dr. Cruickshank CHM 152, Sec Which one of the following would be the best choice as an indicator in the titration of a M ammonia solution (NH 3 ) with a M hydrochloric acid solution (HCl)? [a] thymol blue, K a = [b] chlorophenol blue, K a = [c] thymolphthalein, K a = [d] cresol red, K a = [e] All of these indicators would work equally well. 17. Which of the following has the lowest molar solubility? [a] CaF 2 [b] Sr(IO 3 ) 2 [c] MgF 2 [d] PbCl 2 [e] BaF 2 K sp = K sp = K sp = K sp = K sp = CdCO 3, a slightly soluble substance, will have the lowest solubility in which one of the following solvents or solutions? [a] pure water [b] 0.20 M HCl [c] 0.20 M Na 2 CO 3 [d] 0.10 M Cd(NO 3 ) 2 [e] 0.20 M KCl 19. Which is the relationship between K sp and molar solubility, x, for Al(OH) 3 in water? [a] K sp = 3x 2 [b] K sp = 3x 4 [c] K sp = 9x 5 [d] K sp = 27x 4 [e] K sp = 9x What is the molar solubility of Pb(IO 3 ) 2 in water, at a temperature in which K sp = ? [a] mol/l [b] mol/l [c] mol/l [d] mol/l [e] mol/l 21. What is the molar solubility of PbI 2 in a 0.10 M solution of NaI? [K sp (PbI 2 ) = ]. [a] mol/l [b] mol/l [c] mol/l [d] mol/l [e] mol/l 22. Consider the following reaction: AgBr(s) Ag + (aq) + Br (aq) ΔG = kj Calculate ΔG at 25 C for the reaction when [Ag + ] = M and [Br ] = M. Is the reaction spontaneous or nonspontaneous at these concentrations? [a] ΔG = 40.6 kj, nonspontaneous [c] ΔG = 97.6 kj, nonspontaneous [e] ΔG = 97.6 kj, spontaneous [b] ΔG = 69.1 kj, spontaneous [d] ΔG = 40.6 kj, spontaneous (3)

4 Exam 3 Spring 2006 CHM 152, Sec 2 Dr. Cruickshank PART III: Problems ml of M Ba(NO 3 ) 2 are added to 80.0 ml of M Na 2 CO 3? Will a precipitate of BaCO 3 form? [K sp (BaCO 3 ) = ] [a] Calculate the reaction quotient, Q. [5 pts] BaCO 3 (s) Ba 2+ 2 (aq) + CO 3 (aq) Q sp = [Ba 2+ ] 0 [CO 3 2 ]0 [Ba 2+ ] 0 : M 1 V 1 = M 2 V 2 (20.0 ml)( M) = (100.0 ml)m 2 M 2 = M [CO 3 2 ]0 : M 1 V 1 = M 2 V 2 (80.0 ml)( M) = (100.0 ml)m 2 M 2 = M Q sp = [Ba 2+ ] 0 [CO 3 2 ]0 = ( )( ) = [b] Based on your answer to part (a), will a precipitate of BaCO 3 form? Yes or No. [2 pts] Q > K. The system will shift to the left to reach equilibrium. Yes, a precipitate of BaCO 3 will form g of a mixture containing an unknown monoprotic weak acid (HA) is titrated with 0.21 M NaOH. It takes ml of NaOH to reach the equivalence point. Calculate the mass % of the unknown weak acid in the mixture. The molar mass of the unknown acid is 208 g. Assume that the other component of the mixture does not react with NaOH. [5 pts] g acid mass % acid = 100 g mixture 0.21 mol NaOH L = mol NaOH 1Lsoln 1molHA mol NaOH = mol HA 1molNaOH 208 g HA mol HA = 1.97 g HA 1molHA 1.97 g mass % acid = 100 = 23.8% 8.26 g (4)

5 Spring 2006 Exam 3 Dr. Cruickshank CHM 152, Sec Consider the titration of 40.0 ml of 0.25 M HBr with 0.40 M KOH. [a] What is the initial ph of the solution? (Before any KOH is added.) [2 pts] [H 3 O + ] = 0.25 M ph = 0.60 [b] What is the ph after the addition of 10.0 ml of KOH. [4 pts] HBr + KOH H 2 O + KBr Initial (mol): Change (mol): Final (mol): mol [H3O ] = = 0.12 M L ph = log(0.12) = 0.92 [c] What is the ph after the addition of 25.0 ml of KOH? [2 pts] HBr + KOH H 2 O + KBr Initial (mol): Change (mol): Final (mol): 0 0 Equivalence point, ph = 7. [d] What is the ph after the addition of 40.0 ml of KOH? [4 pts] HBr + KOH H 2 O + KBr Initial (mol): Change (mol): Final (mol): mol [OH ] = L = M poh = 1.12 ph = (5)

6 Exam 3 Spring 2006 CHM 152, Sec 2 Dr. Cruickshank Potentially Useful Information R = J/mol K K = C mol solute grams of substance M= molar mass = Lsoln moles of substance A quadratic equation of the form ax 2 + bx + c = 0, has the solutions: x [products] c = y [reactants] b ± b 4ac x = 2a K K w = [H 3 O + ][OH ] = at 25 C ph = log[h 3 O + ] poh = log[oh ] ph + poh = 14, at 25 C K w = K a K b ph = pk a + log [base] [acid] ΔG = RTlnK ΔG = ΔG + RTlnQ Δ Grxn o = nδgf o (products) nδg f o (reactants) Δ Hrxn o = nδhf o (products) nδh f o (reactants) Δ Srxn o = ns o (products) ns o (reactants) ΔG = ΔH TΔS 2 SOLUBLE COMPOUNDS Compounds containing alkali metal ions (Li +, Na +, K +, Rb +, Cs + ) + and the ammonium ion (NH 4 ) EXCEPTIONS Nitrates (NO 3 ), bicarbonates (HCO3 ), and chlorates (ClO3 ) Halides (Cl, Br, I ) Halides of Ag +, Hg 2 2+, and Pb Sulfates (SO 4 ) Sulfates of Ag, Ca, Sr, Ba, and Pb INSOLUBLE COMPOUNDS EXCEPTIONS 2 3 Carbonates (CO 3 ), phosphates (PO4 ), chromates (CrO4 ), Compounds containing alkali metal ions and and sulfides (S 2 the ammonium ion ) Hydroxides (OH ) Compounds containing alkali metal ions and the Ba 2+ ion (6)