Quarter 3 Practice Exam 2 Answers

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1 Quarter 3 Practice Exam 2 Answers For questions #1-2, choose from the following responses. (A) NH 4 Useful Info: (B) C 5 H 5 N pk a1 (H 2 SO 3 ) = 1.8 pk a2 (H 2 SO 3 ) = (C) HSO 3 pk a (HCN) = 9.2 (D) C 6 H 5 NH 2 pk b (C 5 H 5 N) = 8.8 (E) C 6 H 5 NH 3 pk b (C 6 H 5 NH 2 ) = 9.4 (AB) HCN pk b (NH 3 ) = 4.7 (CD) CN - 1. Which is the strongest acid? E 2. Which is the strongest base? CD 3. Which substance listed below is amphoteric in aqueous solution? (A) HCl (B) NaOH (C) H 3 PO 4 (D) SO 4 2 (E) H 2 AsO 4 4. Which of the following gives the net ionic equation for the reaction between solutions of nitric acid (HNO 3 ) and sodium hydroxide (NaOH)? (A) H (aq) OH (aq) H 2 O(l) (B) H (aq) NaOH(aq) H 2 O(l) Na (aq) (C) HNO 3 (aq) OH (aq) H 2 O(l) NO 3 (aq) (D) HNO 3 (aq) NaOH(aq) H 2 O(l) NaNO 3 (aq) (E) HNO 3 (aq) OH (aq) H (aq) HNO 4 2 (aq) 5. Which of the following statements correctly discusses and explains the difference in acid strength between HF and HCl? (A) HF is stronger because F is more electronegative than Cl. (B) HF is stronger because the bond between H and F is stronger than the bond between H and Cl. (C) HCl is stronger because F is more electronegative than Cl. (D) HCl is stronger because the bond between H and F is stronger than the bond between H and Cl. (E) Both are considered equally strong because they dissociate completely. 6. Which pair of structures below are geometric isomers (a.k.a. stereoisomers)? (E) and 7. Which of the following statements about buffer solutions is NOT true? (A) A buffer solution can be made by reacting one mol of aqueous weak acid with 0.5 mol of strong base. (B) A buffer solution s ph stays roughly constant no matter how much excess acid or base is added. (C) A buffer solution contains a weak acid/base conjugate pair. (D) A buffer solution can be made by reacting one mol of aqueous weak base with 0.5 mol of strong acid. (E) When acid is added to a buffer solution, it reacts with a weak base that is already present.

2 8. Which of the following puts the bases in order from weakest to strongest? (A) weakest base = PO 4 3 < HPO 4 2 < H 2 PO 4 < OH = strongest base (B) weakest base = HPO 4 2 < H 2 PO 4 < PO 4 3 < OH = strongest base (C) weakest base = OH < PO 4 3 < HPO 4 2 < H 2 PO 4 = strongest base (D) weakest base = OH < H 2 PO 4 < HPO 4 2 < PO 4 3 = strongest base (E) weakest base = H 2 PO 4 < HPO 4 2 < PO 4 3 < OH = strongest base 9. A chemist is titrating a solution of ammonia (pk b = 4.7), with a solution of HCl. Which of the following would be the best indicator to identify when the equivalence point has been reached? (4 pt) (A) Methyl orange, pk a = 3.8 (B) Bromothymol blue, pk a = 7.0 (C) Phenolphthalein, pk a = 9.0 (D) Thymolphthalein, pk a = 9.8 (E) None of these would be effective. 10. Which of the following represents the K value for the reaction below? CH 3 NH 2 (aq) H 3 O (aq) CH 3 NH 3 (aq) H 2 O(l) (A) K = K a (CH 3 NH 3 ) (B) K = (C) K = K a (CH 3 NH 3 ) 1 K b (CH 3 NH 2 ) (D) K = K b(ch 3 NH 2 ) (E) K = K b (CH 3 NH 2 ) 14. Formaldehyde (E) 11. Cholesterol (D) 12. Methyl tert-butyl ether (MTBE) AB 15. Citric acid (A) 16. Histamine (B) 13. Progesterone (C)

3 17. the titration of the strongest polyprotic acid? A 18. the titration of the strongest base? D 19. the titration of the weakest monoprotic acid? AB 20. the titration of the most concentrated analyte solution? AB N 2 H 4 (aq) H 2 O(l) N 2 H 5 (aq) OH - (aq) K b = 3.0 x Hydrazine, a weak base, reacts with water according to the reaction represented above. (a) Write the equilibrium constant expression, K b, for the reaction represented above. K b = [N 2H 5 ][OH ] [N 2 H 4 ] (b) A sample of hydrazine is dissolved in water to produce 75.0 ml of a 0.40 M hydrazine solution. Calculate the ph of this solution. mol/l R N 2 H 4 (aq) H 2 O(l) N 2 H 5 (aq) OH - (aq) I C -x x x E 0.4 x 0.4 x x x 2 = x = M poh = log(0.0011) = 2.96 ph = 14-pOH = (c) The solution prepared in part (b) is titrated with 0.40 M HCl. Calculate the ph of the solution when 25.0 ml of the acid has been added. Reaction with a strong acid goes to completion: I ll work in moles because the volume is changing. mol R N 2 H 4 H N 2 H 5 I C E We now have a mixture of acid and conjugate base use H-H equation. First, we need to find the pk a of N 2 H 5. pk a = log(k a ) = log = K b (N 2 H 4 ) ph = pk a log [N 2H 4 ] [N 2 H 5 ] = log V = V

4 (d) What volume of HCl must be added in order to reach the equivalence point of the titration in part (c)? 75.0 ml is needed to reach the equivalence point. Because both solutions have the same concentration, they will reach the equivalence point when equal volumes have been added. (e) The pk a values for several indicators are given below. Which of the indicators listed is most suitable for finding the endpoint of this titration? Justify your answer. Indicator pk a Bromocresol Green 4.6 Bromothymol Blue 7.0 Phenolphthalein 9.3 Because we are dealing with a weak base and a strong acid, when equivalent numbers of moles are present, the ph will be below 7. Therefore, we should choose an indicator that will change colors at a ph lower than 7. An indicator changes color when the ph is roughly equal to its pk a. Therefore, bromocresol green is the best choice, since it will change colors when the ph passes Consider the organic structures shown below. For each one, provide the chemical formula and the systematic name. A B Formula: C 13 H 28 C 8 H 16 Name of A: 4-ethyl-3,7-dimethyl nonane Name of B (ignore cis-trans issues): _3-ethyl-2-hexene 23. When dissolved in water, will each of the compounds below make the ph go up, go down, or remain the same? (a) H 3 PO 4 ph up ph down no change (b) NaHSO 4 ph up ph down no change This one s tricky! HSO 4 could theoretically act as an acid or a base. However, we know that it s conjugate acid is H 2 SO 4, which is strong. So HSO 4 will only act as an acid, giving up H. (c) CsCl ph up ph down no change (d) KOH ph up ph down no change (e) Al(NO 3 ) 3 ph up ph down no change (f) RbC 2 H 3 O 2 ph up ph down no change

5 24. Consider the two acids, HOBr and HOI. (a) Draw a complete Lewis electron dot structure for each acid. (b) Which acid would you expect to be stronger? Explain. HOBr would be a stronger acid. In both cases, the H is bonded to an O atom, so that is not a factor. However, Br is more electronegative than I. Therefore, the electrons in HOBr are pulled a little bit closer to the right end than the electrons in HOI are. Pulling more electron density away from the H makes it more acidic. (c) Consider the reaction below. Would you expect the K value for this reaction to be greater than one, equal to one, or less than one? Explain. (4 pt) HOI(aq) BrO (aq) IO (aq) HOBr(aq) HOI is less likely to give up H than HOBr. Therefore, when equilibrium is reached, there will be more HOI than HOBr. This means that K < Consider the weak acid HSCN. (a) Identify the conjugate base of this acid. SCN (b) Write the chemical equation for the dissociation of this acid. HSCN(aq) H (aq) SCN (aq) (c) Write the equilibrium expression for the K a of this acid. K a = [H ][SCN ] [HSCN] (d) A M solution of HSCN has a ph of Determine the value of K a for HSCN. R HSCN(aq) H (aq) SCN (aq) I C E = K a = [H ][SCN ] [HSCN] = ( )( ) =