CHAPTER 13 Acids and Bases 13.2 The pH Scale

Soil at a high pH makes hydrangea flowers pink Soil at a low pH makes hydrangea flowers blue

pH range pH can be less than 0 for stronger acids Household chemical Acid or base pH ammonia base 11 bar soap 10 baking soda 8.5 soda water acid 4 vinegar 3 lemon juice 2 pH can be less than 0 for stronger acids greater than 14 for stronger bases

pH doesn’t just tell us if a solution is neutral, an acid or a base pH and [H+] pH doesn’t just tell us if a solution is neutral, an acid or a base It also tells us: the concentration of H+ ions in the solution in moles/L Water is neutral: [H+] = 1 x 10-7 M and pH = 7

pH doesn’t just tell us if a solution is neutral, an acid or a base pH and [H+] pH doesn’t just tell us if a solution is neutral, an acid or a base It also tells us: the concentration of H+ ions in the solution in moles/L which is expressed as a power of 10 Water is neutral: [H+] = 1 x 10-7 M and pH = 7

A negative exponent means the number is less than 1 Power of 10 A negative exponent means the number is less than 1

pH and [H+] pH = –log[H+] Definition of pH: Water is neutral: [H+] = 1 x 10-7 M and pH = 7 The number 7 is the logarithm of 0.000 000 1 pH = –log[H+] Definition of pH: Do not forget the “–” sign! logarithm: in base 10, a number A derived from another number B such that 10B=A.

pH and [H+] pH = –log[H+] Definition of pH: Examples: Water is neutral: [H+] = 1 x 10-7 M and pH = 7 The number 7 is the logarithm of 0.000 000 1 pH = –log[H+] Definition of pH: Examples: [H+] = 1 M pH = –log(1) = 0 [H+] = 0.05 M pH = –log(0.05) = 1.3

pH and [H+] pH = –log[H+] Definition of pH: Examples: Water is neutral: [H+] = 1 x 10-7 M and pH = 7 The number 7 is the logarithm of 0.000 000 1 pH = –log[H+] Definition of pH: Examples: Check: [H+] = 1 M pH = –log(1) = 0 [H+] = 10–pH = 10–0 = 1 M [H+] = 0.05 M pH = –log(0.05) = 1.3 [H+] = 10–pH = 10–1.3 = 0.05 M

pH and [H+] A solution of acetic acid (HCH3O2) has an H+ concentration of 5 x 10–5 M. What is the pH of the solution?

pH and [H+] A solution of acetic acid (HCH3O2) has an H+ concentration of 5 x 10–5 M. What is the pH of the solution? Asked: pH of a solution Given: [H+] = 5 x 10–5 M Relationships: pH = –log[H+]

pH and [H+] A solution of acetic acid (HCH3O2) has an H+ concentration of 5 x 10–5 M. What is the pH of the solution? Asked: pH of a solution Given: [H+] = 5 x 10–5 M Relationships: pH = –log[H+] Solve: pH = –log[H+] pH = –log(5 x 10–5) pH = 4.3 Answer: This solution has a pH of 4.3, a relatively weak acid.

pH and [H+] A solution of nitric acid (HNO3) has a pH of 3. What will the pH be if you add 10 mL of the solution to 90 mL of pure water?

pH and [H+] A solution of nitric acid (HNO3) has a pH of 3. What will the pH be if you add 10 mL of the solution to 90 mL of pure water? Asked: pH of the new solution Given: old pH = 3 100 mL of the new solution contains 10 mL of the old solution Relationships: A pH value is a power of 10. A change in 1 pH unit means the concentration changes by a factor of 10.

pH and [H+] A solution of nitric acid (HNO3) has a pH of 3. What will the pH be if you add 10 mL of the solution to 90 mL of pure water? Asked: pH of the new solution Given: old pH = 3 100 mL of the new solution contains 10 mL of the old solution Relationships: A pH value is a power of 10. A change in 1 pH unit means the concentration changes by a factor of 10. Solve: Diluting an acidic solution means the pH increases (fewer H+) The new pH is 4 (not 2). Answer: The new solution has a pH of 4.

pH for bases H2O(l) H+(aq) + OH–(aq) Dissociation of water: [H+] and [OH–] are related

pH for bases Find the pH of a 0.012 M sodium hydroxide (NaOH) solution.

pH for bases Find the pH of a 0.012 M sodium hydroxide (NaOH) solution. Asked: pH of the solution Given: NaOH is a strong base that dissociates 100% in aqueous solution [OH–] = 0.012 M Relationships: pH = 14 + log[OH–]

pH for bases Find the pH of a 0.012 M sodium hydroxide (NaOH) solution. Asked: pH of the solution Given: NaOH is a strong base that dissociates 100% in aqueous solution [OH–] = 0.012 M Relationships: pH = 14 + log[OH–] Solve: pH = 14 + log(0.012) = 14.00 – 1.92 = 12.08 Answer: The solution has a pH of 12.08 and is a strong base.

Measuring pH You can’t measure pH by just looking at a solution, or measuring its density or temperature, but you can measure pH indirectly by: - performing a chemical reaction with a solution of known pH

Measuring pH You can’t measure pH by just looking at a solution, or measuring its density or temperature, but you can measure pH indirectly by: - performing a chemical reaction with a solution of known pH - using a chemical that changes color at different pH values (pH indicators) The color of red cabbage juice at different pH

Measuring pH You can’t measure pH by just looking at a solution, or measuring its density or temperature, but you can measure pH indirectly by: - performing a chemical reaction with a solution of known pH - using a chemical that changes color at different pH values (pH indicators)

Measuring pH You can’t measure pH by just looking at a solution, or measuring its density or temperature, but you can measure pH indirectly by: - performing a chemical reaction with a solution of known pH - using a chemical that changes color at different pH values (pH indicators) - measuring the electrical properties of the solution a pH meter Acids and bases conduct electricity pH and conductivity (flow of electricity) are related

Most acids and bases have a pH between 0 and 14 Finding the pH in acids: pH = –log[H+] Finding the pH in bases: pH = 14 + log[OH–] Measuring the pH involves indirect methods