CHAPTER 16 Acids and Bases The pH Scale

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

The pH Scale Household chemical Acid or basepH ammoniabase11 bar soapbase10 baking sodabase8.5 soda wateracid4 vinegaracid3 lemon juiceacid2 pH range pH can be less than 0 for stronger acids greater than 14 for stronger bases

The pH Scale p H 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 M and pH = 7 pH and [H + ]

The pH Scale p H 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 p ower of 10 Water is neutral: [H + ] = 1 x M and pH = 7 pH and [H + ]

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

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

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

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

The pH Scale A solution of acetic acid (HCH 3 O 2 ) has an H + concentration of 5 x 10 –5 M. What is the pH of the solution? pH and [H + ]

The pH Scale A solution of acetic acid (HCH 3 O 2 ) 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 + ]

The pH Scale A solution of acetic acid (HCH 3 O 2 ) 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 + ]

The pH Scale A solution of nitric acid (HNO 3 ) 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 + ]

The pH Scale A solution of nitric acid (HNO 3 ) 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 = 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 + ]

The pH Scale A solution of nitric acid (HNO 3 ) 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 = 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 and [H + ]

The pH Scale Dissociation of water: H 2 O(l) H + (aq) + OH – (aq) [H + ] and [OH – ] are related pH for bases [H + ][OH - ] = 1 x Becomes: pH + pOH = 14

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

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

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

The pH Scale 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

The pH Scale Measuring pH The color of red cabbage juice at different 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 pH Scale 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 pH Scale 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 Measuring pH Acids and bases conduct electricity pH and conductivity (flow of electricity) are related a pH meter

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

The pH Scale [H + ] = 10 -pH K w = [H + ] [OH – ] [H + ] [OH – ] = 1 x Finding the [H + ] in acids: Finding the [OH - ] in bases: All of the values, pH, pOH, [H + ] and [OH – ] can be calculated using our formulas: