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Calculating the pH of Acids and Bases

Calculating the pH of Acids and Bases. Strong vs. Weak. Strong Acids & Bases. Dissociate completely in water Also known as strong electrolytes Electrolytes conduct electricity in aqueous solutions The more ions dissociated…the more electricity conducted. Strong Acids & Bases. HCl HNO 3

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Calculating the pH of Acids and Bases

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  1. Calculating the pH of Acids and Bases Strong vs. Weak

  2. Strong Acids & Bases • Dissociate completely in water • Also known as strong electrolytes • Electrolytes conduct electricity in aqueous solutions • The more ions dissociated…the more electricity conducted

  3. Strong Acids & Bases • HCl • HNO3 • HClO4 • H2SO4 • All alkali metal hydroxides

  4. Weak Acids & Bases • Do not completely dissociate in water • The less dissociated they are…the weaker electrolytes they are • Any acid or base not on the aforementioned list is considered weak • Weak bases are often difficult to recognize…look for the presence of –NH2, the amine group

  5. Note the one-way arrow! pH of Strong Acids • Write the dissociation of HCl HCl(aq)  H+(aq) + Cl-(aq) • Let’s figure out the pH of a 12M solution of concentrated HCl.

  6. pH of Strong Acids • Make a chart for the dissociation

  7. pH of Strong Acids

  8. pH of Strong Acids

  9. pH of Strong Acids

  10. pH of Strong Acids • If the [H+] is 12M, then we can determine the pH of the solution • pH = -log [H+] • pH = -log [12] • pH = -1.08 • Since it dissociates completely and an equilibrium is never reached, you really don’t need to make a chart.

  11. pH of Strong Acids • What is the pH of concentrated sulfuric acid, 18M? • This is a diprotic acid…that is, it has two dissociable hydrogen ions • Only one H+ dissociates at a time. • H2SO4(aq)  H+(aq) + HSO41-(aq) • HSO41-(aq)  H+(aq) + SO42-(aq)

  12. pH of Strong Acids • The first dissociation is complete, while the second is not. The second reaches equilibrium • Thus, the first dissociation really determines the pH of a strong, multiprotic acid. • pH = -log [18] • pH = -1.255

  13. Note the one-way arrow! pH of Strong Bases • Write the dissociation of NaOH NaOH(aq)  Na+(aq) + OH-(aq) • Let’s figure out the pH of a 6M solution of NaOH.

  14. pH of Strong Bases • Make a chart for the dissociation

  15. pH of Strong Bases

  16. pH of Strong Bases

  17. pH of Strong Bases

  18. pH of Strong Bases • If the [OH-] is 6M, then we can determine the pH of the solution • pOH = -log [OH-] • pOH = -log [6] • pOH = -0.778 • pH + pOH = 14 • pH = 14.8 • pH = 14 – (-0.778)

  19. Note the two-way arrow! pH of Weak Acids • Write the dissociation of the weak acid HC2H3O2 HC2H3O2(aq)  H+(aq) + C2H3O2-(aq) • Let’s figure out the pH of a 17.4M solution of concentrated HC2H3O2.

  20. pH of Weak Acids • Since it’s weak, it will reach equilibrium. • Since it will reach equilibrium, it has an equilibrium constant. • The equilibrium constant of a weak acid is called a Ka. • Write the Ka expression for the dissociation of acetic acid.

  21. pH of Weak Acids • Ka = [H+][C2H3O2-] [HC2H3O2] • The Ka value for acetic acid is 1.76 x 10-5 M • Make a chart for the dissociation

  22. pH of Weak Acids

  23. pH of Weak Acids

  24. pH of Weak Acids

  25. pH of Weak Acids • Plug into the Ka expression • 1.76 x 10-5 = [x][x] [17.4 - x] • Test the 5% rule • x = 0.0175

  26. pH of Weak Acids • x = 0.0175 = [H+] • pH = -log [0.0175] • pH = 1.76

  27. Note the two-way arrow! pH of Weak Bases • Write the dissociation of the weak base NH3 NH3(g) + H2O(l)  NH4+(aq) + OH-(aq) • When dissociating a weak base, react it with water to justify the acceptance of the H+

  28. pH of Weak Bases • Since it’s weak, it will reach equilibrium. • Since it will reach equilibrium, it has an equilibrium constant. • The equilibrium constant of a weak base is called a Kb. • Write the Kb expression for the dissociation of ammonia.

  29. pH of Weak Bases • Kb = [NH4+][OH-] [NH3] • The Kb value for ammonia is 1.75 x 10-5 M • Make a chart for the dissociation of a 15.3M solution of NH3 in water.

  30. pH of Weak Bases

  31. pH of Weak Bases

  32. pH of Weak Bases

  33. pH of Weak Bases • Plug into the Kb expression • 1.75 x 10-5 = [x][x] [15.3 - x] • Test the 5% rule • x = 0.0164

  34. pH of Weak Bases • x = 0.0164 = [OH-] • pOH = -log [0.0164] • pOH = 1.79 • pH = 14 - 1.79 • pH = 12.2

  35. pH of Multiprotic Weak Acids • Write the dissociations of tartaric acid, H2C4H4O6 (found in cream of tartar) H2C4H4O6(aq)  H+(aq) + HC4H4O61- (aq) HC4H4O61-(aq)  H+(aq) + C4H4O62- (aq)

  36. 5g x 1mol x 1 = 0.333M 150.1g 0.1L pH of Multiprotic Weak Acids • Let’s figure out the pH of a 100-mL solution that contains 5.00g of H2C4H4O6. Ka1 is 9.20 x 10-4 and Ka2 is 4.31 x 10-5. • First determine the initial molarity of the tartaric acid. • Make a chart for the 1st dissociation.

  37. pH of Multiprotic Weak Acids

  38. pH of Multiprotic Weak Acids

  39. pH of Multiprotic Weak Acids

  40. pH of Multiprotic Weak Acids • Write the Ka1 expression Ka1 = [H+][HC4H4O61-] [H2C4H4O6] • Plug your values into the expression. 9.20 x 10-4 = [x][x] [0.333 - x] • Test the 5% rule.

  41. pH of Multiprotic Weak Acids • Use the quadratic to solve for x. x = 0.0170 = [H+] • More hydrogen ion will dissociate in the next dissociation…this is just the amount from the first dissociation. • Make a chart for the 2nd dissociation.

  42. pH of Multiprotic Weak Acids

  43. pH of Multiprotic Weak Acids

  44. pH of Multiprotic Weak Acids

  45. pH of Multiprotic Weak Acids

  46. pH of Multiprotic Weak Acids • Write the Ka2 expression Ka2 = [H+][C4H4O62-] [HC4H4O61-] • Plug your values into the expression. 4.31 x 10-5 = [0.0170 + x][x] [0.0170 - x] • Test the 5% rule.

  47. pH of Multiprotic Weak Acids • 5% rule works! x = 4.31 x 10-5 = [C4H4O62-] • So the total amount of hydrogen ion is represented by 0.0170 + 4.31 x 10-5… • [H+] = 0.0170431 • pH = 1.77

  48. pH of Multiprotic Weak Acids • What are the equilibrium concentrations of all of the species? [H2C4H4O6]eq = 0.333 – 0.017 = 0.316 M [HC4H4O61-]eq = 0.017 – 4.31 x 10-5 = 0.01696 M [C4H4O62-]eq = 4.31 x 10-5M

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