291 Version Exam 2 Holcombe (52460) This print-out should have 33 questions. Multiple-choice questions may continue on the next column or page nd all choices before answering. Some constants: Kw=1e-14; R=8.314 J/Kmol 001 10.0 points When dissolved in water, bases produce 1. salts. 2. H+ ions. 3. OH ions. correct 4. hydronium ions. 5. H3 O+ ions. Explanation: Bases produce hydroxide ions (OH ) in water; acids produce hydronium ions (H3 O+ ). 002 10.0 points A diprotic acid H2 A has values of Ka1 = 1.0 106 and Ka2 = 1.0 1010 . What is the [A2 ] in a 0.10 M solution of H2 A? 1. 1.0 1010 M correct 2. 3.2 106 M 3. 0.10 M 4. 0.20 M 5. 3.2 104 M Explanation: 003 10.0 points How many grams of HNO3 are needed to make 315 mL of a solution with a pH of 2.60? The molar mass of HNO3 is 63.02 g/mol. 1. 19.9 g 2. 0.158 g 3. 1.25 105 g 4. 0.0498 g correct 5. 0.502 g Explanation: pH = 2.6 MM = 63.02 g/mol
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V = 315 mL = 0.315 L
[H+ ] = 10pH = 102.6 = 0.00251189 Since HNO3 is a strong acid, One HNO3 one H+ , so (0.315 L) HNO3 (aq) H+ (aq) + NO (aq) . 3
[H+ ] = [HNO3 ] = 0.00251189 mol and 0.00251189 mol HNO3 1L = 0.000791244 mol HNO3 .
Thus 63.02 g HNO3 (0.000791244 mol HNO3 ) 1 mol HNO3 = 0.0498642 g HNO3 004 10.0 points
What would be the pH of a 1 M solution of Na2 HPO4 ? Assume H3 PO4 has a pKa1 of 2 and a pKa2 of 7 and a pKa3 of 13. 1. 10 correct 2. 13 3. 7.5 4. 4.5 5. 2 6. 7 Explanation: For a solution composed of a single amphoteric species (HPO4 2 ), pH = 0.5(pKa2 + pKa3 ) = 0.5(7 + 13) = 10
Version 291 Exam 2 Holcombe (52460) 005 10.0 points A solution containing an unspecied amount of HCl and ammonia (NH3 ) has a pH of 8.81 What is the ratio of NH3 to NH+ in the 4 solution? (For NH3 , Kb = 1.8 105 ) 1. 0.233 2. 0.359 correct 3. 2.06 4. cannot be determined 5. 0.485 6. 2.79 7. 5.6 1010 5. pH = 4.74
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Explanation: [base] = 0.05 M As mentioned, this is a weak base, so use the equation to calculate weak base [OH ] concentration: [OH ] = = Kb Cb (6.6 109 ) (0.05)
= 1.81659 105
After nding [OH ], you can nd pH using either method below: A) pOH = log 1.81659 105 = 4.74074 pH = 14 4.74074 = 9.25926 or B) Kw = [H+ ][OH ] = 1 1014 Kw [H+ ] = [OH ] 1 1014 = 5.50482 1010 = 5 1.81659 10 pH = log 5.50482 1010 = 9.25926 007 10.0 points What is the pH of a 0.020 M solution of hydrosulfuric acid, a diprotic acid? Ka1 = 1.1 107 Ka2 = 1.0 1014 1. 9.67
Explanation: Consider this equilibria... NH+ NH3 + H+ 4 Rearranging the equilibrium expression gives... Ka [NH3] = = ratio +] [H [NH4+] [H+] = 108.81 = 1.54882 109 M Ka = Kw /1.8 105 = 5.556 1010
5.556 1010 = 0.359 ratio = 1.54882 109 M Note that the same answer could be obtained by using the base equilibria and Kb 006 10.0 points You have a weak molecular base with Kb = 6.6 109 . What is the pH of a 0.0500 M solution of this weak base? 1. None of these 2. pH = 7.12 3. pH = 3.63 4. pH = 9.26 correct
2. 4.69 3. 3.65 4. 4.33 correct 5. 5.22 6. 7.84 7. 7.00
Version 291 Exam 2 Holcombe (52460) Explanation: Solve using ONLY the 1st ionization. So this works like any other monoprotic acid where the assumption [H+ ] = (Conc)(Ka1 ) is valid. 008 10.0 points The pH of a solution changes from 5.5 to 3.5. This indicates that the [OH ] of the solution has 1. increased by a factor of 100. 2. increased by a factor of 2. 3. decreased by a factor of 100. correct 4. decreased by a factor of 2. 5. decreased by 2 M. 6. increased by 2 M. Explanation: 009 10.0 points A solution has [H+ ] = 1.5 107 M. What is the [OH ] in this solution? 1. 6.7 107 M 2. 6.7 106 M 3. 6.7 108 M correct 4. None of these 5. 1.5 107 M
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Calculate the [OH ] concentration in a 0.5 M solution of potassium uoride (KF). Ka for HF is 7.2 104 . 1. 3.6 1011 M 2. 0.5 M 3. 2.64 106 M correct 4. 6.94 1012 M 5. 6.0 106 M Explanation: 011 10.0 points
The pH of 0.1 M (C3 H7 NH2 ) propylamine aqueous solution was measured to be 11.86. What is the value of pKb of propylamine? 1. 3.35 2. 2.14 3. 11.86 4. 3.25 correct 5. 0.11 6. 0.96 Explanation: M = 0. 1 M pH = 11.86 Analyzing the reaction with molarities, C3 H7 NH2 + H2 O C3 H7 NH+ + OH 3 0. 1 0 0 x x x 0. 1 x x x pOH = 14 11.86 = 2.14 . [C3 H7 NH+ ] = [OH ] = 10pOH 3 = 102.14 = 0.00724436 mol/L
Explanation: [H+ ] = 1.5 107 M Kw = [H+ ][OH ] = 1.0 1014 Kw [OH ] = + [H ] 1.0 1014 = 6.7 108 M = 1.5 107 010 10.0 points
Version 291 Exam 2 Holcombe (52460) The Kb is [C3 H7 NH+ ][OH ] 3 Kb = [C3 H7 NH2 ] (0.00724436)2 = 0.1 0.00724436 = 0.000565796 . and the pKb is pKb = log(0.000565796) = 3.24734 . 012 10.0 points Consider the fractional composition diagram for phosphoric acid. 1.0 Fractional Composition 0.8 0.6 0.4 0.2 2 4 6 8 10 12 14
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Based on its dot structure, the best classication for O2 is as 1. an Arrhenius base. 2. an Arrhenius acid. 3. a Lewis base. correct 4. a Bronsted Lowry acid. 5. a Lewis acid. Explanation: The oxygen anion has a complete octet as its dot structure shows. Since it can only make available a share in an electron pair, it must be a Lewis base. 014 10.0 points A solution of 0.2 M boric acid is prepared as an eye wash. What is the approximate pH of this solution? For boric acid Ka = 7.2 1010 . 1. pH = 3 2. pH = 7 3. pH = 5 correct 4. pH = 4 5. pH = 6 Explanation:
pH What is the approximate pH of a solution prepared by adding several grams of Na2 HPO4 to a few hundred mL of water? 1. 10 correct 2. 2 3. 12 4. 7 5. 5 Explanation: Na2 HPO4 is the salt of the monoprotic species which is the dominate species between pHs of 7.2 and 12.7 which is the orange or shortest dashed curve. The best estimate of the pH is the dead center of that range which is a pH of 10 which is the average of pK2 and pK3 for the acid. 013 10.0 points
015 10.0 points Would the value of Kw change if the temperature were raised from 25 C to 50 C? 1. Yes correct 2. No Explanation: 016 10.0 points If 17.6 g of HCl are used to produce a 14.5 L solution, what is the pH of the solution? 1. 11.2
Version 291 Exam 2 Holcombe (52460) 2. 3.01 3. 0.316 4. 1.48 correct 5. 2.54 Explanation: m = 17.6 g V = 14.5 L To nd the pH we need to calculate the concentration of H+ : 1 mol HCl ? M H = 17.6 g HCl 36.46 g + 1 1 mol H 1 mol HCl 14.5 L = 0.0333 M H+
+
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A water solution of NaI will exhibit a pH value of 1. about 7. correct 2. less than 7. 3. zero. greater 4. than 7. Explanation: 019 10.0 points What is the concentration of SO2 in 2.0 M 4 H2 SO4 ? Ka1 is strong and Ka2 = 1.2 102 . 1. 1.2 102 M correct 2. 4.0 101 M 3. 2.0 101 M 4. 1.0 107 M 5. 4.0 102 M Explanation: 020 10.0 points Which of the following is a correct mass balance expression for the addition of H2 CO3 to water? 1. CH2 CO3 = [HCO ] + [CO2 ] 3 3 2. CH2 CO3 = [H2 CO3 ] + [HCO ] + [CO2 ] 3 3 correct 3. Kw = [H+ ] + [OH ] 4. [H+ ] = [HCO ] + [CO2 ] + [OH ] 3 3 Explanation: 021 10.0 points Consider the [H3 O+ ] in each of the following solutions. For which system is the percent dissociation to produce H3 O+ the greatest? Do not go through the calculations.
pH = log[0.0333] = 1.48 017 10.0 points 3 A 1.0 10 M NH3 solution is prepared. What is the pH of the solution? 1. 3 or less 2. between 7 and 11 correct 3. 7 4. 11 or more 5. between 3 and 7 Explanation: NH3 is a weak base, and will not generate 100 percent OH ions. The solution would be more basic than a neutral solution, and so the pH should be greater than 7. The solution will be less basic than strong base solution of the same concentration, and so the pH should be less than 11 since a 1.0 103 M strong base solution would have a hydroxide ion concentration of 1.0 103 and a pH of 11. 018 10.0 points
Version 291 Exam 2 Holcombe (52460) 1. 1.00 M NH3 , K = 1.8 105 2. 0.010 M HCN, K = 4.0 1010 3. 0.010 M H2 S, K1 = 1.0 10 4. 0.010 M (COOH)2 , correct
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2. indicates an H+ concentration of 0.0176 M. 3. indicates an H+ concentration of 0.176 M. 4. is impossible because pH cannot be negative. 5. indicates an H+ concentration of 31.6 M. correct 6. indicates an H+ concentration of 0.0316 M. Explanation: [H+ ] = 10pH = 10(1.5) = 31.6228 M pH CAN be negative! 024 10.0 points Carbonic acid (H2 CO3 ) is a diprotic acid with Ka1 = 4.2 107 and Ka2 = 4.8 1011 . The ion product for water is Kw = 1.0 1014 . What is the [H3 O+ ] concentration in a saturated carbonic acid solution that is 0.037 molar? 1. 4.2 107 M 2. 3.7 1020 M 3. 6.5 104 M 4. 7.4 102 M 5. 1.2 104 M correct Explanation: 025 10.0 points The conjugate base of H2 SO4 is: 1. H2 O 2. H3 SO+ 4 3. H2 SO3
K1 = 5.9 102
Explanation: The general reaction for a weak acid is HA + H2 O H3 O+ + A [H3 O+ ][A ] . [HA] The acid for which Ka is the greatest will have the greatest percent dissociation. All of the given species are weak acids, with the exception of NH3 . Since NH3 is a weak base its value of [H3 O+ ] will be less than that of the acids. Ka = 022 10.0 points Which acid has the strongest conjugate base? 1. HBr 2. HI 3. HCl 4. HF correct Explanation: HBr, HCl and HI are strong acids which completely ionize in water so their conjugate base is a poor proton acceptor. HF is a weak acid which is only partially ionized in water. Its conjugate base F is better able to bond with a proton. 023 10.0 points Complete the following statement. A pH value of 1.5 1. indicates an H+ concentration of 0.0316 M.
5. 0.010 M H2 O2 , K = 2.4 1012
Version 291 Exam 2 Holcombe (52460) 4. SO2 4 8. CO2 ; CO2 3 3
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5. OH 6. H3 O+ 7. HSO correct 4 Explanation: Remove one H+ ion from a species to nd its conjugate base. 026 10.0 points What would be the pH of a 109 M solution of HCl? 1. 6.954 2. 6.921 3. 6.876 4. 6.998 correct Explanation: For a dilute strong +2 + [H ] Ca[H ] Kw = 0 b b2 4ac + [H ] = x = 2a pH = 6.998 027 10.0 points The conjugate acid of HCO is ? 3 conjugate base of HCO is ? . 3 1. H+ ; OH 2. H2 CO3 ; CO2 correct 3 3. H2 CO3 ; H2 O 4. H+ ; CO2 3 5. CO2 ; H2 CO3 3 6. H2 CO3 ; H2 CO3 7. CO2 ; OH 3 acid,
Explanation: To produce a conjugate acid, HCO must 3 be acting as a base. Therefore it is a proton acceptor forming H2 CO3 . To produce a conjugate base, it must be acting as an acid. Therefore it is a proton donator forming CO2 . 3 028 10.0 points Sodium uoride is added to pure water and stirred to dissolve. Compared to pure water, the new solution is 1. acidic pH 2. neutral pH 3. basic pH correct 4. More than one of these is true. 5. unchanged pH Explanation: Sodium uoride is a basic salt and therefore changes solutions to a more basic pH. 029 10.0 points Consider ve generic acids (HA, HB, HC, HD, and HE) that have the following ionization constants.
and the Acid HA HB HC HD HE Ionization Constant Ka value 3.6 103 8.3 104 2.6 106 9.3 106 7.3 107
Which of the following anions will be the WEAKEST base? 1. B 2. A correct
Version 291 Exam 2 Holcombe (52460) 3. E 4. C 5. D Explanation: The larger the Ka , the greater the dissociation and the stronger the acid. In general, a conjugate base is opposite in strength from its parent acid strength. HA is the strongest acid listed (largest Ka ), therefore A is the weakest base. 030 10.0 points Rank following acids from most to least acidic: NH+ Ka = 1.35 1010 4 N2 H+ Ka = 7.94 109 5 CH3 NH+ Ka = 2.34 1011 3 (CH3 )3 NH+ Ka = 1.8 1010 1. (CH3 )3 NH
+
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6. 103 M, NaOH. Explanation: HCl is the only strong acid so all the HCl present will produce H3 O+ . 103 M HCl has a higher concentration than 105 M HCl. 032 10.0 points What is the pH of a solution made by dissolving 0.100 mole of sodium acetate (NaCH3COO) in enough water to make one liter of solution? Ka for CH3 COOH is 5 1.80 10 . 1. 7.46 106 2. 1.82 106 3. 3.75 4. 5.13 5. 9.25 6. 1.80 106 7. 8.87 correct 8. 5.56 1011 9. 10.25 10. 5.56 1010 Explanation: 033 10.0 points A solution at a pH of 1.0 contains the weak acid HA and its conjugate base A . What percent of the acid is dissociated? (Ka = 106 and [HA] + [A ] = 103 M). 1. < 10% correct 2. 100% 3. between 10% and 90% 4. between 90% and 100%
> CH3 NH+ > N2 H+ 3 5 > NH+ 4
> NH+ 4
+
2. CH3 NH+ > N2 H+ 3 5
> (CH3 )3 NH
3. N2 H+ >(CH3 )3 NH+ > NH+ > CH3 NH+ 5 4 3 correct 4. NH+ > (CH3 )3 NH+ > CH3 NH+ > N2 H+ 5 3 4 Explanation: Explanation: The value of Ka is directly proportional to the acidity (strength) of the acid. By ranking from the largest (7.94 109) to the smallest (2.34 1011 ) Ka, one is ranking from the most to least acidic. 031 10.0 points Select the solution with the lowest pH 1. 103 M, acetic acid. 2. 103 M, HCl. correct 3. 105 M, HCl. 4. 105 M, acetic acid. 5. 103 M, Ca(OH)2 .
Version 291 Exam 2 Holcombe (52460) 5. Not enough information is given. Explanation:
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