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Chapter 19.3

pH. Chapter 19.3. Vocabulary Word. hydronium ion: H 3 O + hydroxide ion: OH -. Ionization of Water. When water ionizes it is acting as both an acid and a base in the same reaction H 2 O  H + + OH - K eq = [H + ] [OH - ] [H 2 O]. Ionization of Water.

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Chapter 19.3

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  1. pH Chapter 19.3

  2. Vocabulary Word • hydronium ion: H3O+ • hydroxide ion: OH-

  3. Ionization of Water • When water ionizes it is acting as both an acid and a base in the same reaction H2O  H+ + OH- Keq = [H+] [OH-] [H2O]

  4. Ionization of Water Keq [H2O] = Kw = [H+] [OH-] • At 298K experimentally the Keq of water is 1.0 x 10 -14 • Kw = 1.0 x 10 -14 • [H+] = [OH-] = 1.0 x 10 -7

  5. Using Kw to Calculate [H+] or [OH-] • If [H+] = 1.0 x 10 -5, what is [OH-]? Kw = [H+] [OH-] 1.0 x 10 -14 = 1.0 x 10 -5 [OH-] 1.0 x 10 -14 = [OH-] 1.0 x 10 -5 [OH-] = 1.0 x 10 -9

  6. Acid or Base 1.0 x 10 -5 > 1.0 x 10 -9 [H+] > [OH-] Therefore the solution is an acid.

  7. pH Scale • Since the concentrations of H+ and OH- are so small, chemists adapted a simpler way to express them on a log scale, pH. • pH = - log [H+]

  8. Vocabulary Word • pH = - log [H+], values range from 0 to 14. Acids have a pH less than 7. Bases have pH greater than 7.

  9. Log Scales • Since pH is measured on a log scale, each unit is a 10 fold change in [H+]. • A difference in pH of 2 is a 100 fold change in [H+] • A difference in pH of 5 units is a 100,000 change in [H+]

  10. Calculating Acidity • pH can also be calculated from the [H+] • If [H+] = 1.0 x 10 -2 • pH = - log 1.0 x 10 -2 = 2.0

  11. Calculating Basicity • Sometimes chemists calculate basicity instead • pOH is a measure of basicity • pOH = - log [OH-]

  12. Vocabulary Word • pOH: - log [OH-], values range from 0 to 14. Acids have a pOH of greater than 7 and bases have a pOH of less than 7.

  13. Calculating pH or pOH • pH + pOH = 14 • If [OH-] = 4.0 x 10 -3, calculate the pH and the pOH. • given [OH-], so pOH = - log 4.0 x 10 -3 • pOH = 2.4 • pH = 14 - 2.4 = 11.6

  14. Calculating [H+] or [OH-] • If the pH of a solution is 3.5, calculate [H+] and [OH-] • given pH, so 3.5 = - log [H+] • [H+] = antilog -3.5 • [H+] = 3.2 x 10 -4

  15. Calculating [H+] or [OH-] • If pH = 3.5, pOH = 14 - 3.5 = 10.5 • pOH = - log [OH-] • 10.5 = - log [OH-] • antilog -10.5 = [OH-] • [OH-] = 3.16 x 10 -11

  16. Measuring pH • Litmus can distinguish acids from bases • acids turn litmus red • bases turn litmus blue • A quantitative value of pH can be measured using: • indicator paper (pH paper) • a pH meter

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