Chem 112, Fall 05 Exam 3A

Transcription

1 Before you begin, make sure that your exam has all 10 pages. There are 32 required problems (3 points each, unless noted otherwise) and two extra credit problems (3 points each). Stay focused on your exam. YOU MUST: Put your name and student ID on the bubble sheet correctly. Put all your answers on the bubble sheet; nothing on this exam will be used for grading. Sign the statement on the last page of the exam. Turn in both the exam and bubble sheet when you are done. Good Luck! Some values of Ksp Carbonates BaCO x 10-9 NiCO x 10-9 Hydroxides Fe(OH) x Fe(OH) x Ni(OH) x Sulfates BaS x Sulfides CaS 8 x 10-6 NiS 3.0 x page 1

2 1. According to the Brønsted-Lowry definition, a base A. increases the H 3 O + concentration in an aqueous solution. B. increases the OH - concentration in an aqueous solution. C. is a proton acceptor. D. is a proton donor. E. is an electron-pair donor. 2. Which of the following is never a Brønsted-Lowry acid in an aqueous solution? A. hydrogen chloride, HCl(g) B. dihydrogen sulfide, H 2 S(g) C. ammonium chloride, NH 4 Cl(s) D. hydrogen fluoride, HF(g) E. magnesium oxide, MgO(s) 3. Which salt forms a 0.10 M aqueous solution with the highest ph? A. NaNO 3 B. NH 4 Cl C. FeCl 3 D. Ca(ClO 4 ) 2 E. NaF 4. Consider the following series of acids: CH 3 CO 2 H, ClCH 2 CO 2 H, Cl 2 CHCO 2 H, Cl 3 CCO 2 H Which statement explains the trend in the pk A s (left to right). A. pk A s increase due to inductive effects generated by the substitution of H with Cl. B. pk A s decrease due to inductive effects generated by the substitution of H with Cl. C. pk A s increase due to the larger number of resonance structures possible in the anion. D. pk A s decrease due to the larger number of resonance structures possible in the anion. E. Both B and D. page 2

3 5. At 15 C, the water ionization constant, K w, is What is the H 3 O + concentration in neutral water at this temperature? A M B M C M D M E. 2.2 M 6. If you mix equal molar quantities of HF (K a = ) and KCN (K b = ), the resulting solution will be A. acidic because K a of HF is greater than K b of CN -. B. basic because K a of HF is greater than K a of HCN. C. basic because K a of HF is greater than K b of CN -. D. basic because K b of F - is less than K b of CN -. E. neutral because the weak acid neutralizes the weak base. 7. Determine the equilibrium constant for the reaction below. (K a (HF) = , K a (NH 4 + ) = ) HF(aq) + NH 3 (aq) NH 4 + (aq) + F - (aq) A B C D E Which of the following chemical equations corresponds to K a3 for phosphoric acid? A. HPO 4 2- (aq) + H 2 O(l) PO 4 3- (aq) + H 3 O + (aq) B. PO 4 3- (aq) + H 2 O(l) HPO 4 2- (aq) + OH - (aq) C. H 3 PO 4 (aq) + H 2 O(l) H 2 PO 4 - (aq) + H 3 O + (aq) D. H 3 PO 4 (aq) + 3 H 2 O(l) PO 4 3- (aq) + 3 H 3 O + (aq) E. H 2 PO 4 - (aq) + H 2 O(l) HPO 4 2- (aq) + H 3 O + (aq) page 3

4 9. Boric acid (H 3 BO 3 ) has a pk a value of What is the K b of sodium borate (NaH 2 BO 3 ) at 25 o C? A B C D E In the following acid-base reaction equilibrium C 9 H 7 NH + + C 6 H 5 COO - C 9 H 7 N + C 6 H 5 COOH A. C 9 H 7 NH + is the conjugate acid of C 9 H 7 N. B. C 6 H 5 COO - is the conjugate acid of C 6 H 5 COOH. C. C 6 H 5 COOH and C 6 H 5 COO - represent a conjugate acid-base pair D. Both A and B E. Both A and C 11. Suppose that K < 1 for the equilibrium in problem 10. A. C 6 H 5 COO - is the weaker base and C 9 H 7 NH + is the stronger acid B. C 6 H 5 COO - is the weaker base and C 9 H 7 NH + is the weaker acid C. C 9 H 7 N is the weaker base and C 6 H 5 COOH is the weaker acid D. C 9 H 7 N is the stronger base and C 6 H 5 COOH is the stronger acid E. Both B and D 12. (4 pts) For the following three reactions, K > 1 HF (aq) + C 6 H 5 COO - (aq) F - (aq) + C 6 H 5 COOH (aq) HC 9 H 7 O 4 (aq) + C 6 H 5 COO - (aq) C 6 H 5 COOH (aq) + C 9 H 7 O 4 - (aq) HF (aq) + C 9 H 7 O 4 - (aq) F - (aq) + HC 9 H 7 O 4 (aq) Among the three acids and three conjugate bases, which are the strongest acid and base? A. HC 9 H 7 O 4 and C 6 H 5 COO - B. HF and C 6 H 5 COO - C. HF and C 9 H 7 O 4 - D. C 6 H 5 COOH and F - E. C 9 H 7 CO 4 - and F - page 4

5 13. Two solutions of the same weak acid (pk A = 6) were made to have different concentrations (10-4 M and 0.1 M). The 10-4 M solution was found to have a A. smaller ph than the 0.1 M solution B. larger ph than the 0.1 M solution C. the same ph as the 0.1 M solution D. Not enough information is provided to determine if a difference in ph exists. 14. Two buffer solutions were made with equal molar mixtures of a weak acid its conjugate base, and differed only by the total concentration of acid plus base. The buffer solution with the larger total acid plus base concentration had A. a ph that was larger than the buffer with the smaller total concentration. B. a ph that was smaller than the buffer with the smaller total concentration. C. a ph that was the same as the buffer with the smaller total concentration. D. Not enough information is provided to make this decision. 15. Each of the following mixtures can produce a buffer solution EXCEPT A. HClO 4 and NaClO 4 B. HF and NaF C. NaHCO 3 and Na 2 CO 3 D. Na 2 HPO 4 and Na 3 PO 4 E. NH 4 Cl and NH Which of the following mathematical expressions is the Henderson-Hasselbalch equation? A. p Ka = ph + log [ conjugate base] [ acid] B. - OH ph = p Ka + log + HO 3 C. ph = p Ka + log [ acid] [ conjugate base] D. p Ka = ph - log [ acid] [ conjugate base] E. ph = p Ka + log [ conjugate base] [ acid] page 5

6 17. What is the ph of a solution that results from adding 25 ml of 0.15 M HCl to 25 ml of 0.52 M NH 3? (K b of NH 3 = ) A B C D E Which of the following acid-base pairs is most suited to make a buffer with a ph = 7.68? pk A pk B A. HCO 2 H/NaHCO B. H 2 CO 3 /NaHCO C. HOCl/NaOCl D. H 3 BO 3 /NaH 2 BO E. NH 4 Cl/NH ml of a buffer solution formed with NH 4 Cl and NH 3 has a ph = 9.64 and the [NH 3 ] = 0.5 M. What is the number of grams of NH 4 Cl required to make this solution? (MW NH3 = 17.0 g/mol, MW NH4Cl = 53.5 g/mol) (K b of NH 3 = ) A g B g C g D g E g 20. If the ratio of acid to base in a buffer increases by a factor of 10, the ph of the buffer A. decreases by 1 B. decreases by 10 C. increases by 10 D. increases by 1 E. remains unchanged 21. What is the ph of the buffer that results when 11 g of NaCH 3 CO 2 is mixed with 85 ml of 1.0 M CH 3 CO 2 H and diluted with water to 1.0 L? (K a of CH 3 CO 2 H = ; MW of CH 3 CO 2 Na = g/mol) A B C D E page 6

7 22. Which one of the following conditions is always met at the equivalence point of the titration of a monoprotic weak base with a strong acid? A. The ph of the solution is equal to B. The volume of acid added from the buret equals the volume of base titrated. C. The molarity of the acid equals the initial molarity of the weak base. D. The percent ionization of the acid equals the percent ionization of the base. E. The moles of acid added from the buret equals the initial moles of weak base. 23. Which one of the following conditions is always true for a titration of a weak acid with a strong base? A. A colored indicator with a pk a less than 7 should be used. B. If a colored indicator is used, it must change color rapidly in the weak acid s buffer region. C. Equal volumes of weak acid and strong base are required to reach the equivalence point. D. The equivalence point occurs at a ph equal to 7. E. The equivalence point occurs at a ph greater than A 25.0 ml sample of M NH 3 (aq) is titrated with M HCl(aq). What is the ph at the equivalence point? (K b of NH 3 = ) A B C D E Which of the following equations is the solubility product for magnesium iodate, Mg(IO 3 ) 2? A. B. C. D. E. K K K K K sp sp = [Mg ][I ] [O ] = [Mg ][I ] [3O ] = [Mg ][IO ] 2+ - sp 3 = [Mg ] [IO ] sp 3 = [Mg ][IO ] sp 3 page 7

8 26. The solubility of SrSO 4 in water is g in 1.0 L at 25 C. What is the value of K sp for SrSO 4? A B C D E What is the molar solubility of Fe(OH) 3 (s) in a solution that is buffered at ph 2.75 at 25 C? The K sp of Fe(OH) 3 is at 25 C. A mol/l B mol/l C mol/l D mol/l E mol/l 28. The following anions can be separated by precipitation as silver salts: Cl -, Br -, I -, CrO If Ag + is added to a solution containing the four anions, each at a concentration of 0.10 M, in what order will they precipitate? Compound K sp AgCl Ag 2 CrO AgBr AgI 1.5 x A. AgCl Ag 2 CrO 4 AgBr AgI B. AgI AgBr Ag 2 CrO 4 AgCl C. Ag 2 CrO 4 AgCl AgBr AgI D. Ag 2 CrO 4 AgI AgBr AgCl E. AgI AgBr AgCl Ag 2 CrO 4 page 8

9 29. (4 pts) Barium sulfite (s) and barium fluoride (s) are in equilibrium with a solution containing 8.70 x 10-3 M ammonium fluoride (completely soluble). Calculate the concentration of sulfite ion present in this solution. K sp (BaSO 3 ) = 6.0 x 10-7, K sp (BaF 2 ) = 1.7 x A M B M C. 2.7 x 10-5 M D. 3.1 x 10-3 M E. 6.0 x 10-7 M 30. A solution contains 0.10 M potassium sulfide and 0.10 M sodium carbonate. Solid nickel acetate is added slowly to this mixture. What substance precipitates first? A. K 2 CO 3 B. NiS C. CH 3 CO 2 K D. NiCO 3 E. No precipitate will form. 31. The ionic solids AgNO 3 and KI are dissolved in water and mixed together and in various proportions to form solutions with the Ag + and I - concentrations listed below: [Ag + ] (M) [I - ] (M) Which of these solutions will form a precipitate? (K sp = 1.5 x ) A. 1 B. 1 and 3 C. 1 and 2 D. all three E. None of the solutions form precipitate. page 9

10 32. (4pts) A solution contains 6.8 x 10-4 M KOH. Solid iron(iii) nitrate is added slowly to this mixture. What is the concentration of iron(iii) ion when precipitation first begins? A. 6.8 x 10-4 M B. 1.2 x M C. 1.4 x M D. 2.0 x M E. 1.7 x 10-8 M EXTRA CREDIT 33. (3 pts) At 25 C, only 1.04 x 10-3 g Cu(NO 3 ) 2 will dissolve per liter of a solution that is buffered at ph What is the value of K sp for Cu(OH) 2? The molar mass of Cu(NO 3 ) 2 is g/mol. A B C D E (3 pts) What is the concentration of PO 4 3- in a solution created by mixing equal volumes of 0.1 M NaH 2 PO 4 and 0.2 M Na 2 HPO 4? (The acid dissociation constants for phosphoric acid are K a1 = , K a2 = , and K a3 = ) A M B. 0.2 M C D E. zero Please sign the following statement at the completion of the exam. I, (print name), have completed this exam honestly, and pledge not to discuss it with anyone who has not taken the exam until it is graded and returned. (signature) page 10

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