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Chapter 19

Chapter 19. The pH Scale. H 2 O ( l ) + H 2 O ( l ). H 3 O + ( aq ) + OH − ( aq ). Autoionization of Water. As we have seen, water is amphoteric. In pure water, a few molecules act as bases and a few act as acids. It breaks apart into + an – part This is referred to as autoionization.

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Chapter 19

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  1. Chapter 19 The pH Scale

  2. H2O(l) + H2O(l) H3O+(aq) + OH−(aq) Autoionization of Water • As we have seen, water is amphoteric. • In pure water, a few molecules act as bases and a few act as acids. • It breaks apart into + an – part • This is referred to as autoionization.

  3. I. Calculating pH -log [H+] or –log [H3O+] A. pH= B. pOH= -log [OH-] 14 C. pH + pOH = Neutral pH 7 14 0 Acidic Basic  pOH 0 14

  4. A. The pH Scale • Because the pH scale is a log scale based on 10, the pH changes by 1 for every power of 10 change in the [H+].

  5. Review

  6. pH of Common Substances

  7. Calculating the pH pH = - log [H+] (Remember that the [ ] mean Molarity) Example: If [H+] = 1 X 10-10pH = - log 1 X 10-10 pH = - (- 10) pH = 10 Example: If [H+] = 1.8 X 10-5pH = - log 1.8 X 10-5 pH = - (- 4.74) pH = 4.74

  8. pH calculations – Solving for H+ If the pH of Coke is 3.12, [H+] = ??? Because pH = - log [H+] then - pH = log [H+] Take antilog (10x) of both sides and get 10-pH =[H+] [H+] = 10-3.12 = 7.6 x 10-4 M *** to find antilog on your calculator, look for “Shift” or “2nd function” and then the log button

  9. pH calculations – Solving for H+ • A solution has a pH of 8.5. What is the Molarity of hydrogen ions in the solution? pH = - log [H+] 8.5 = - log [H+] -8.5 = log [H+] Antilog -8.5 = antilog (log [H+]) 10-8.5 = [H+] 3.16 X 10-9 = [H+]

  10. The pH of a Solution • pH = log[H3O+] and [H3O+] = 10pH • Acidic pH < 7.00 • Neutral pH = 7.00 • Basic pH > 7.00 E.g. determine the pH of a solution in which [H3O+] = 5.40x106 M E.g.2 determine the pH of a solution in which the [OH] = 3.33x103 M E.g.3 determine the pOH of a solution in which the [OH] = 3.33x103 M E.g.4 Determine the [H3O+] if the pH of the solution is 7.35. • The term pX is defined in exactly the same way as pH. Eg.5 What is the pCa if [Ca2+] = 6.44x10-4

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