Chapter 15 Exam 3

Exam 3 (Ch 15) 1. (12 points) T F At the equivalence point, all titrations will have a pH of 7 T F A buffer solution is created during a weak acid/strong base titration. T F A buffer solution is created during a weak base/strong acid titration T F A buffer solution is created during a strong base/strong acid titration 2. (4 points) What type of titration will have an equivalence point at a pH greater than7? Explain (be specific). A weak acid / strong base will have a pH >7 at the equivalence point. At this point, all the weak acid has been converted to a weak base giving a basic solution. 3. Assume you are titrating a 10.00mL sample of 0.055M HI with 0.091M KOH. Answer the following questions about the titration:  (4 points) What is the pH of the acid before adding any KOH? -log(0.055) = 1.26  (8 points) What is the pH after adding 1.00mL of KOH? 0.055molHI 0.01000L HI  0.00055molHI 1 L HI 0.00100L KOH 0.090molKOH  0.000090mol KOH 1 L KOH 4.6x10 4 mol  0.042M 0.01100L OH+ H+  b 9.0x10-5mol 5.5x10-4mol Δ -9.0x10-5mol -9.0x10-5mol a 0 4.6x10-4mol H2O pH = -log(0.042) = 1.38 4. (4 points) Determine the pH of 100.mL of a buffer solution containing 0.20M of HClO and 0.20M NaClO. Ka of HClO is 3.2x10-8. pKa = -log(3.2x10-8) = 7.49 pH  7.49  log 0.20  7.49 0.20 5. (4 points) If you had 500.mL of the same buffer solution as above (at the same concentration), which of the following would be true? a. The pH would be higher b. The pH would be lower c. The pH would be the same d. Not enough information to determine 6. (7 points) What would be the pH after adding 3.00mL of 3.0M HCl to the 100.mL of a buffer solution containing 0.20M of HClO and 0.20M NaClO? 0.100L HClO 0.00300L HCl Since both parts of the buffer HA/A- have the same concentration, they will both have 0.020mol. No need to do a separate calculation 0.20molHClO  0.020molHClO 1 L HClO 3.0molHClO  0.0090molHCl 1 L HClO H+ + ClOb 9.0x10-3mol 2.0x10-2mol Δ -9.0x10-3mol -9.0x10-3mol a 0 1.1x10-2mol  HClO 2.0x10-2mol +9.0x0-3mol 2.9x10-2mol pH  7.49  log 0.011  7.07 0.029 7. (4 points) If you had 500.mL of the same buffer solution (at the same concentration as above), and you added the same amount of HCl (3.0mL of 3.0M), which of the following would be true? a. The resulting pH would be higher than what I calculated b. The resulting pH would be lower than what I calculated c. The resulting pH would be the same as what I calculated d. Not enough information to determine e. pH = pka 8. (7 points) How many mL of 3.0M HCl would need to be added to make the buffer “run out” (stop acting like a buffer)? Assume you start with 100.mL buffer containing 0.20M of HClO and 0.20M NaClO. 0.100L ClO- 0.20molClO- 1 mol HCl 1 L HCl  6.7x10 -3 L HCl  6.7mL 1 mol ClO 3.0molHCl 1 L ClO 9. (4 points) If you had 500.mL of the same buffer solution as above (at the same concentration), and you added enough HCl to make the buffer “run out”, which of the following would be true? a. It would take a smaller volume of HCl to make the buffer “run out” b. It would take a larger volume of HCl to make the buffer “run out” c. It would take the same volume of HCl to make the buffer “run out” d. Not enough information to determine 10. (2 points) Write the reaction showing how Mg(OH)2 behaves in water (to simplify the equation, do not show H2O). Remember, in CHM151, we called Mg(OH)2 “insoluble” but now we call it a “sparingly soluble” salt. 11. (12 points) How will the solubility of Mg(OH)2 be affected if each of the following are performed?  Add more Mg(OH)2  Solubility will: increase decrease stay the same Solubility will: increase decrease stay the same Add NaOH  Add H2SO4 Solubility will:  increase decrease stay the same increase decrease stay the same Add Mg(NO3)2 Solubility will: 12. (4 points) In class, teacher took a sample of Cu(NO 3)2 and added ammonia, NH3. Initially, there was a precipitate of Cu(OH)2. However, as more NH3 was added, the Cu(OH)2 eventually dissolved. Explain why. The initial precipitate formed because the NH3 made the solution basic. By adding a lot more NH3, a complex ion formed with the Cu2+. Complex ions are soluble. 13. (4 points) Assuming the graph to the right is a titration of a weak acid with a strong base, at which point in the graph would the pH = pKa of the acid? A B C D E 14. (4 points) Which of the following containers best represents the chemical species in the solution at point “D” in the titration curve? 15. (4 points) Which of the containers from above will have a pH larger than the pKa of the weak acid being titrated? (choose all that apply) __x_A __x_B ____C ____D __x_E 16. (4 points) Which of the following salts would be most soluble in pure water? a. b. c. d. e. AgCl, AuI, CuCN, MnS, TlBr, Ksp = 1.8 x 10-10 Ksp = 1.6 x 10-23 Ksp = 3.2 x 10-20 Ksp = 3.0 x 10-15 Ksp = 3.4 x 10-6 17. (4 points) Which of the following mixtures would make the best buffer? a. b. c. d. e. CH3CO2H and NH4Cl HCl and NaOH NaCH3COO and NH3 NaCH3COO and NH4Cl NH3 and NH4Cl 18. (4 points) Calculate the Ksp of a compound with the formula MX2 if the molar solubility is 1.0x10-9M. Ksp = [M2+][X-]2 Extra Credit: Determine the concentrations of 1.0M HCN/CN- in a buffer solution with at a pH of 8.5. Ka = 5.0x10-10.