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AP Chemistry Unit 10 – Acids and Bases

AP Chemistry Unit 10 – Acids and Bases. Lesson 4 – The pH Scale Book Section: 16.4. “p” as an Operator. Last time, we learned that the autoionization constant, K w , for water, was equal to 1.0 x 10 -14 . [H + ][OH - ] = 1.0 x 10 -14 = K w

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AP Chemistry Unit 10 – Acids and Bases

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  1. AP ChemistryUnit 10 – Acids and Bases Lesson 4 – The pH Scale Book Section: 16.4

  2. “p” as an Operator • Last time, we learned that the autoionization constant, Kw, for water, was equal to 1.0 x 10-14. • [H+][OH-] = 1.0 x 10-14 = Kw • Acidity and basicity can be measured more generally without scientific notation using the operator “p” • An operator is any mathematical process (multiplication, division, raising to a power, etc. are all operators) • “p” operator = negative logarithm • pX = -log(X)

  3. pH and pOH • Therefore, the terms “pH and pOH” refer to a logarithmic scale (like the Richter scale) • pH = -log[H+] = -log[H3O+] • pOH = -log[OH-] • A pH of 2 is ten times more acidic than a pH of 3. (logarithmic scale) • Because [H+][OH-] = 1.0 x 10-14, • pH + pOH = 14

  4. The pH Scale • The pH scale runs from 1 to 14. • 1 is most acidic • 7 is neutral • 14 is most basic • Conversely, pOH runs on the same scale • 1 is most basic • 7 is neutral • 14 is most acidic

  5. pH Examples • Lemon juice: 2.0 • Tomatoes: 4.0 • Coffee: 5.0 • Baking soda: 8.0 • Limewater: 10.0 • Ammonia (household): 12.0

  6. Summary of pH Formulas • pH = -log[H+] 10-pH = [H+] • pOH = -log[OH-] 10-pOH = [OH-] • pH + pOH = 14 • [H+][OH-] = Kw = 1.0 x 10-14

  7. Practice Problem • Calculate the pH value for a solution in which [OH-] is 0.010 M and a solution in which [OH-] is 1.8 x 10-9M.

  8. Practice Problem • Calculate the pH value for a solution in which [OH-] is 0.010 M and a solution in which [OH-] is 1.8 x 10-9M. • pH = -log(1.0 x 10-12) = 12.00 • pH = -log(5.6 x 10-6) = 5.25

  9. Practice Problem • A sample of freshly squeezed apple juice has a pH of 3.76. Calculate [H+].

  10. Practice Problem • A sample of freshly squeezed apple juice has a pH of 3.76. Calculate [H+]. • [H+] = 10-3.76 = 1.7 x 10-4M • pH and Pretzels

  11. HW: 16.36, 16.38, 16.40, 16.42 • This week: • Wednesday: Strong Acids and Bases (16.5) • Thursday: Titration 2: Molar Mass of Solid Acids (Cerami Out) • Friday: Titration 2: Molar Mass of Solid Acids (Cerami Out) • Heat of Formation of MgO – Due Tuesday

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