Hydrogen Ions and Acidity 19.2 To test a diagnosis of diabetic coma, a doctor orders several tests, including the acidity of the patient’s blood. Results from this test will be expressed in units of pH. You will learn how the pH scale is used to indicate the acidity of a solution and why the pH scale is used.

19.2 The pH Concept The pH Concept How is the hydrogen-ion concentration used to classify a solution as neutral, acidic, or basic?

The pH of pure water or a neutral aqueous solution is 7.0. 19.2 The pH Concept A solution in which [H+] is greater than 1  10–7 M has a pH less than 7.0 and is acidic. The pH of pure water or a neutral aqueous solution is 7.0. A solution with a pH greater than 7 is basic and has a [H+] of less than 1  10–7 M.

19.2 The pH Concept The hydrogen-ion concentrate of a solution is used to classify the solution as acidic, neutral, or basic. INTERPRETING GRAPHS a. Identify What is [H3O+] in a neutral solution? b. Describe How does [H3O+] compare with [OH-] in an acidic solution? c. Compare and Contrast In terms of ion concentrations, how are basic solutions different from acidic solutions?

19.2 The pH Concept

pH and Significant Figures 19.2 The pH Concept pH and Significant Figures The pH scale shows the relationship between pH and the hydrogen-ion concentration. Interpreting Diagrams What happens to [H+] as pH increases?

19.2 Measuring pH Measuring pH What is the most important characteristic of an acid-base indicator?

19.2 Measuring pH An indicator is a valuable tool for measuring pH because its acid form and base form have different colors in solution.

Phenolphthalein changes from colorless to pink at pH 7–9. 19.2 Measuring pH Phenolphthalein changes from colorless to pink at pH 7–9. Acid-base indicators respond to pH changes over a specific range. Phenolphthalein changes from colorless to pink at pH 7–9.

Universal Indicators 19.2 Measuring pH You can find acidic and basic substances in your home. a) Universal indicator solution has been added to solutions of known pH in the range from 1 to 12 to produce a set of reference colors. b) Universal indicator has been added to samples of vinegar, soda water, and ammonia solution. Interpreting Photographs Use the reference colors to assign pH values to vinegar, soda water, and ammonia solution.

19.2 Measuring pH pH Meters A pH meter provides a quick and accurate way to measure the pH of a solution. a) Water is neutral, having a pH of 7. b) The pH of vinegar, a dilute aqueous solution of ethanoic (acetic) acid, is about 3. c) The pH of milk of magnesia, an aqueous suspension of magnesium hydroxide, is 10.5. Applying Concepts What are some advantages of using a pH meter rather than an indicator?

A solution in which the hydroxide ion concentration is 1 x 10 -5 is 19.2 Measuring pH Sample Questions: A solution in which the hydroxide ion concentration is 1 x 10 -5 is a. Acidic b. Basic c. Neutral d. None of the above A pH meter provides a quick and accurate way to measure the pH of a solution. a) Water is neutral, having a pH of 7. b) The pH of vinegar, a dilute aqueous solution of ethanoic (acetic) acid, is about 3. c) The pH of milk of magnesia, an aqueous suspension of magnesium hydroxide, is 10.5. Applying Concepts What are some advantages of using a pH meter rather than an indicator?

Which of these solutions is most basic? a. [H+] = 1 x 10 -11 19.2 Measuring pH Sample Questions: Which of these solutions is most basic? a. [H+] = 1 x 10 -11 b. [OH-] = 1 x 10-4 c. [H+] = 1 x 10 -2 d. [OH-] = 1 x 10 -13 A pH meter provides a quick and accurate way to measure the pH of a solution. a) Water is neutral, having a pH of 7. b) The pH of vinegar, a dilute aqueous solution of ethanoic (acetic) acid, is about 3. c) The pH of milk of magnesia, an aqueous suspension of magnesium hydroxide, is 10.5. Applying Concepts What are some advantages of using a pH meter rather than an indicator?

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