Acid base MC review 2015 key

AP Chemistry Name……………………………………...…….. Acid Base Review Hour……. Directions: Complete the following problems. Use the answers below to answer Questions 1 – 3. An answer can be used once, more than once or not at all. a. Br-­‐(aq) b. S2-­‐(aq) c. NH4+(aq) d. HI(aq) 1. Would be classified as the conjugate acid of a weak base. C the weak base being NH3 2. Would be classified as the conjugate base of a strong acid. A the strong acid is HBr 3. Would be classified as a strong acid. D Select from the answers below for Questions 4 – 6. An answer can be used once, more than once or not at all. a. The number of moles present per liter b. The extent of the dissociation of the electrolytes (solute) c. Inter-­‐ionic attractions 4. Which of the above is most closely related to the distinction between strong acids and weak acids? C strong acids completely dissociate 5. Which of the above is most closely related to the distinction between concentrated and dilute acid solutions. A concentrated solutions have a larger number of moles per liter (molarity) 6. Which of the above accounts for a pH of 2 for 0.01 M HCl, but a pH of 4 for 0.0001 M HCl? A both are strong acids but HCl has more moles of H+ per liter. 7. A 1.0 L solution contains equal molar amounts of NH4Cl and NH3. The balanced, net ionic that occurs in the buffered solution is ___. (Kb for NH3 = 1.8×10-­‐5). a. NH4+(aq) + H2O(l) ⇄ NH3(aq) + H3O+(aq) b. NH4Cl(aq) + NH3(l) ⇄ 2NH3(aq) + HCl(aq) c. NH3(aq) + H2O(l) ⇄ NH4+(aq) + OH-­‐(aq) d. NH4Cl(aq) + H2O(l) ⇄ NH4OH(aq) + HCl(aq) NH3 is a weak base and NH4+ is its conjugate acid. As Kb > Ka the buffer will be basic. Choice C is correct Page 1 of 7 8. Adding distilled water to the buffered solution in Question 7 will ___ the pH of the solution. a. increase b. decrease c. not affect Adding water to the buffer to the reaction will not change the pH. In the H-­‐H equation the ratio of conjugates will not change as both will be diluted equally. Use the answers below to answers Questions 9 – 11. An answer can be used once, more than once or not at all. a. KOH(aq) b. NaHSO4(aq) c. NaCl(aq) d. NH3(aq) 9. Which of the above solutions is an example of a Brønsted-­‐Lowry acid? B HSO4-­‐ acts as an acid 10. Which of the above aqueous solutions would be classified as neither a Brønsted-­‐Lowry acid or a base? C Na+ is from a strong base NaOH. Cl-­‐ is from the strong acid HCl. NaCl will be a neutral salt. 11. Which of the above 0.10 M aqueous solutions would have the highest pH? A KOH is a strong base. Use the answers below to answer Questions 12 – 14. An answer can be used once, more than once or not at all. Consider 100 mL of each solution, a. 0.1 M NaOH (a strong base) b. 0.2 M NaCl (a neutral pH salt) c. 0.1 M NaC2H3O2 (a basic salt as C2H3O2-­‐ + H2O ⇄ HC2H3O2(aq) + OH-­‐) d. 0.1 M NH4Cl (an acidic salt as NH4+ + H2O ⇄ NH3 + H3O+) 12. Which solution would be expected to have the lowest pH? D 13. Which solution would have a pH closest to pH = 8.2? C NaOH is a strong base with a pH of 13 (pOH = -­‐log (0.10) = 1 so pH = 14 – 1 = 13). NaC2H3O2 is the only other base. 14. Which solution pH will remain the same if 50 mL of distilled water is added to each substance? B NaCl is a neutral solution. Page 2 of 7 Use the answers below to answer Questions 15 – 18. An answer can be used once, more than once or not at all. a. Hydrazoic acid, HN3 (Ka = 1.9 × 10-­‐5) b. Hydrofluoric acid, HF (Ka = 6.8 × 10-­‐4) c. Nitrous acid, HNO2 (Ka = 4.5 × 10-­‐4) d. Phenol, C6H5OH (Ka = 1.3 × 10-­‐10) e. Aniline, C6H5NH2 (Kb = 4.3 × 10-­‐10) 15. Which compound is best when preparing a buffer of pH = 9.5? E Only base in the list. Plus -­‐log (4.3 × 10-­‐10) = 9.37 16. Which 0.20 M solution will have the lowest pH? B HF has the largest Ka and is therefore is the most acidic. 17. Which 0.10 M solution will have the lowest percentage ionization? D Phenol is the weakest (in this case acid) of all the selections. 18. Rank the compounds in order of increasing acid strength, lowest to highest. a. D < E < A < C < B b. E < D < A < C < B c. D < E < A < B < C d. B < C < A < E < D Exactly 100 mL of four 0.10 M monoprotic acid solutions are prepared. A pH meter is used to measure their pH at 25°C. Use the table below to answer Questions 19 – 22. An answer can be used once, more than once or not at all. Choice Acid pH a. HW 6.50 b. HX 3.20 c. HY 2.10 d. HZ 4.75 19. Which acid has the largest Ka? The acid with the largest Ka will have the lowest pH as it will have the largest dissociation and the largest [H+]. As there are equal volumes of equal acid concentrations it can be determined that acid HY has the largest Ka. Choice C. 20. Which sodium salt (e.g. NaW) has the largest pH? The worst acid will have a conjugate with the most basic properties. Acid HW is the least acidic (lowest Ka) and thus its conjugate W-­‐ will be the most basic. Choice A. 21. Which acid has the largest equivalence point pH when titrated with 0.10 M NaOH(aq)? The equivalence point contains the conjugate base and water. W-­‐ is the strongest conjugate (see 20) thus HW and NaOH will have the largest equivalence point. Choice A. Page 3 of 7 22. Which acid will require the largest volume of 0.10 M NaOH(aq) to reach the equivalence point? All are equal. All have equal moles of HA and thus will require equal volumes. 23. When 0.10 M solutions of the solutes: HClO4, NH4Br, KOH and KCN are arranged in order of increasing [H+], the correct order is a. KOH < KCN < NH4Br < HClO4 b. KCN < KOH < HClO4 < NH4Br c. HClO4 < NH4Br < KCN < KOH d. NH4Br < HClO4 < KOH < KCN Recall that HClO4 is a strong acid! KOH is a strong base, NH4+ is an acidic cation and CN-­‐ is a basic anion. 24. The sodium salt, NaA, of a weak acid is dissolved in water and no other substance is added. Which of the following statements is correct? a. [H+] = [A-­‐] c. [A-­‐] = [OH-­‐] b. [H+] = [OH-­‐] d. [HA] = [OH-­‐] A-­‐ + H2O ⇄ HA + OH-­‐ As reaction shifts right [HA] and [OH-­‐] increase equally. 25. These acids are listed in order of decreasing acid strength in water. HI > HNO2 > CH3COOH > HCN According to the Brønsted-­‐Lowry theory, which anion is the weakest base? a. I-­‐ c. CH3COO-­‐ -­‐ b. NO2
d. CN-­‐ Strong acid = weak conjugate base. HI is a strong acid! 26. The conjugate acid of the bicarbonate ion, HCO3-­‐ in H2O is a. H3O+ c. OH-­‐ 2-­‐
b. CO3 d. H2CO3 HCO3-­‐ + H2O ⇄ H2CO3 + OH-­‐ Base Acid C. Acid C. Base 27. Which 0.10 M solution has the highest pH? In other words…which salt is the most basic! a. sodium carbonate…basic! b. sodium chloride…neutral c. ammonium carbonate…difficult to tell…need to compare Ka and Kb. Will be close to neutral. d. ammonium chloride…acidic Page 4 of 7 28. Which is the strongest acid? a. Acetic acid – (Ka = 1.8 × 10-­‐5) b. Benzoic acid – (Ka = 6.3 × 10-­‐5) c. Formic acid – (Ka = 1.8 × 10-­‐4) d. Nitrous acid – (Ka = 6.0 × 10-­‐4) 29. According to Brønsted-­‐Lowry theory, which of these species cannot be amphoteric? a. NH4+(aq) c. NH2-­‐(aq) b. NH3(aq) d. NH2-­‐(aq) Amphoteric means it can accept and donate H+. NH4+ cannot ACCEPT a proton as addition would violate octet rule. 30. What is the order of concentration of the ions and molecules in a nitrous acid solution? Nitrous acid, HNO2, is a weak acid. a. H3O+ = NO2-­‐ > HNO2 > OH-­‐ b. H3O+ = NO2-­‐ = HNO2 = OH-­‐ c. HNO2 > H3O+ = NO2-­‐ > OH-­‐ d. HNO2 > NO2-­‐ > H3O+ > OH-­‐ HNO2 is a weak acid so not much will dissociate. [HNO2] will be the largest. In the equilibrium HNO2 + H2O ⇄ NO2-­‐ + H3O+… [NO2-­‐] = [H3O+] by stoichiometry. This solution is acidic so [H3O+] > [OH-­‐]. 31. Which titration will have an equivalence point at the lowest pH? In other words more acidic! a. HCl and KOH (KCl at equivalence, a neutral salt, pH = 7) b. HF and NaOH (NaF at equivalence, a basic salt as F-­‐ + H2O ⇄ HF + OH-­‐, large pH) c. HF and NH3 (NH4F at equivalence, NH4+ conjugate of a weak base NH3, F-­‐ conjugate of a weak acid, pH will be close to 7) d. HCl and NH3 (NH4Cl at equivalence, an acidic salt as NH4+ + H2O ⇄ H3O+ + NH3, low pH) 32. Which pair of chemicals would produce a buffer solution when equal numbers of moles of each are mixed? a. HF and NaF (F-­‐ and HF are conjugates and HF is a weak acid) b. HF and HC2H3O2 c. HCl and NH3 d. HNO2 and NaOH 33. Which of these solutions, appropriately combined, could be used to produce a buffer? a. HCl and NaOH b. HNO2 and NaNO3 c. KI and KOH d. HF and NaOH (if fewer moles of OH-­‐ than HF would result in HF and F-­‐) Page 5 of 7 34. Which pair constitutes a buffer? a. HCl and KCl b. NaOH and NaCl c. HNO2 and NaNO2 (HNO2 is a weak acid and its conjugate is NO2-­‐) d. HNO3 and NH4NO3 35. A 0.100 M solution of acetic acid (Ka = 1.8 × 10-­‐5) is titrated with a 0.100 M solution of NaOH. What is the pH when 50% of the acid has been neutralized? a. 2.38 c. 5.70 b. 4.74 d. 7.00 This is the half equivalence point, at this point [HA] = [A-­‐]… ⎛ [A% ] ⎞
pH#=#%log#K a +#log ⎜
so...pH#=#%log(1.8 × 10 %5 )#=#4.74 ⎟
⎝ [HA] ⎠
36. Which indicator is most appropriate for titrating a 0.100 M solution of NH3 with 0.100 M HNO3? The equilibrium constant, Kb, for NH3 equals 1.8 × 10-­‐5 Indicator pH color transition range a. cresol red 1-­‐2 b. methyl red 4-­‐6 c. phenolphthalein 8-­‐10 d. alizarin yellow 10-­‐12 Equivalence will be NH4NO3. NO3-­‐ is the conjugate of a strong acid, HNO3. NH4+ is the conjugate of a weak base, NH3. Solution will be acidic as NH4+ + H2O ⇄ NH3 + H3O+. 0.100 M solutions are not very concentrated thus pH will be acidic but closer to 7 than 1. Page 6 of 7 Questions 37 – 40 refer to the titration of NH3(aq) with HCl(aq). The titration curve that results is shown below. An answer can be used once, more than once, or not at all. 37. The point of maximum buffering. B. This occurs where there is a large concentration of the conjugates. Point B is the half equivalence point, the point where the conjugates are the largest (and equal). 38. The concentration of the conjugate is at its maximum molar concentration. C. The equivalence point contains the conjugate and water. 39. The equivalence point. C. 40. The point where half of the base has been neutralized. B. The half equivalence point. Page 7 of 7