P H Scale And Calculations

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pH is a measure of the acidity or basicity of a solution. It is defined as the cologarithm of the activity of dissolved hydrogen ions (H+). Hydrogen ion activity coefficients cannot be measured experimentally, so they are based on theoretical calculations. The pH scale is not an absolute scale; it is relative to a set of standard solutions whose pH is established by international agreement.

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P H Scale And Calculations

  1. 1. pH Scale and Calculations Chapter 14 Page 565-574
  2. 2. pH Scale • We use this scale to measure the strength of an acid or base. • pH is defined as the –log[H+] • pH can use the concentration of hydronium ions or hydrogen ions.
  3. 3. 7 Acid Base 0 pH Scale 14 Zumdahl, Zumdahl , DeCoste, World of Chemistry 2002, page 515
  4. 4. pH of Common Substances Timberlake, Chemistry 7th Edition, page 335
  5. 5. pH of Common Substance pH [H1+] [OH1-] pOH 14 1 x 10-14 1 x 10-0 0 NaOH, 0.1 M 13 1 x 10-13 1 x 10-1 1 Household bleach 12 1 x 10-12 1 x 10-2 2 More basic Household ammonia Lime water 11 1 x 10-11 1 x 10-3 3 Milk of magnesia 10 1 x 10-10 1 x 10-4 4 Borax 9 1 x 10-9 1 x 10-5 5 Baking soda Egg white, seawater 8 1 x 10-8 1 x 10-6 6 Human blood, tears Milk 7 1 x 10-7 1 x 10-7 7 Saliva Rain 6 1 x 10-6 1 x 10-8 8 More acidic Black coffee 5 1 x 10-5 1 x 10-9 9 Banana Tomatoes 4 1 x 10-4 1 x 10-10 10 Wine Cola, vinegar 3 1 x 10-3 1 x 10-11 11 Lemon juice 2 1 x 10-2 1 x 10-12 12 Gastric juice 1 1 x 10-1 1 x 10-13 13 0 1 x 100 1 x 10-14 14
  6. 6. Acid – Base Concentrations 10-1 pH = 3 pH = 11 concentration (moles/L) H3O+ OH- pH = 7 10-7 H3O+ OH- OH- H3O+ 10-14 [H3O+] > [OH-] [H3O+] = [OH-] [H3O+] < [OH-] acidic neutral basic Timberlake, Chemistry 7th Edition, page 332 solution solution solution
  7. 7. pH pH = -log [H+] Kelter, Carr, Scott, Chemistry A World of Choices 1999, page 285
  8. 8. Self-Ionization Of Water • Even the purest of water conducts electricity. This is due to the fact that water self-ionizes, that is, it creates a small amount of H3O+ and OH-. H2O + H2O  H3O+ + OH- Kw = [H3O+][OH-] • Kw - ion product of water Kw = 1.0 x 10-14 at 25 oC • This equilibrium constant is very important because it applies to all aqueous solutions - acids, bases, salts, and non-electrolytes - not just to pure water.
  9. 9. Self ionization reaction of water: + - O O O H + H H H + O H H H H   2 H2O  H3 O OH   K w [H3O ] [OH ] 10 -14 (at 25 C)  Kw [H 3O ]  [OH-]
  10. 10. pH and pOH • pH = - log[H3O+] [H3O+] = 10-pH pOH = - log[OH-] [OH-] = 10-pOH • pKw = pH + pOH = 14.00 • neutral solution: [H3O+] = [OH-] = 10 –7 M pH = 7.0 acidic solution: [H3O+] > 10-7 M pH < 7.0 basic solution: [H3O+] < 10-7 M pH > 7.0
  11. 11. Practice • Finish the following for homework Page 566 #12-15 (using Kw) Page 569 # 16-19 (using pH = -log[H+] Page 572 # 20-23 Page 578 # 2, 6-8

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