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Understanding Autoionization of Water and pH Calculations

Learn about autoionization of water, ion product of water, acidic and basic solutions, pH calculations, and practice problems in introductory chemistry. Discover how to determine pH, pOH, and the nature of solutions.

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Understanding Autoionization of Water and pH Calculations

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  1. Day Three 14.8-14.9 Tro's Introductory Chemistry, Chapter 14

  2. Autoionization of Water • Water is actually an extremely weak electrolyte. • Therefore, there must be a few ions present. • About 1 out of every 10 million water molecules form ions through a process called autoionization. H2O Û H+ + OH– H2O + H2O Û H3O+ + OH– • All aqueous solutions contain both H3O+ and OH–. • The concentration of H3O+ and OH– are equal in water. • [H3O+] = [OH–] = 1 x 10-7M at 25 °C in pure water. Tro's Introductory Chemistry, Chapter 14

  3. Ion Product of Water • The product of the H3O+ and OH– concentrations is always the same number. • The number is called the ion product of water and has the symbol Kw. • [H3O+] x [OH–] = 1 x 10-14 = Kw. • As [H3O+] increases, the [OH–] must decrease so the product stays constant. • Inversely proportional. Tro's Introductory Chemistry, Chapter 14

  4. Acidic and Basic Solutions • Neutral solutions have equal [H3O+] and [OH–]. • [H3O+] = [OH–] = 1 x 10-7 • Acidic solutions have a larger [H3O+] than [OH–]. • [H3O+] > 1 x 10-7; [OH–] < 1 x 10-7 • Basic solutions have a larger [OH–] than [H3O+]. • [H3O+] < 1 x 10-7; [OH–] > 1 x 10-7 Tro's Introductory Chemistry, Chapter 14

  5. Example—Determine the [H3O+] for a 0.00020 M Ba(OH)2 and Determine Whether the Solution Is Acidic, Basic, or Neutral. Tro's Introductory Chemistry, Chapter 14

  6. Practice—Determine the [H3O+] Concentration and Whether the Solution Is Acidic, Basic, or Neutral for the Following: • [OH–] = 0.000250 M • [OH–] = 3.50 x 10-8 M • Ca(OH)2 = 0.20 M Tro's Introductory Chemistry, Chapter 14

  7. pH • The acidity/basicity of a solution is often expressed as pH. • pH = ─log[H3O+], [H3O+] = 10−pH • Exponent on 10 with a positive sign. • pHwater = −log[10-7] = 7. • Need to know the [H+] concentration to find pH. • pH < 7 is acidic; pH > 7 is basic; pH = 7 is neutral. Tro's Introductory Chemistry, Chapter 14

  8. pH, Continued • The lower the pH, the more acidic the solution; the higher the pH, the more basic the solution. • 1 pH unit corresponds to a factor of 10 difference in acidity. • Normal range is 0 to 14. • pH 0 is [H+] = 1 M, pH 14 is [OH–] = 1 M. • pH can be negative (very acidic) or larger than 14 (very alkaline).

  9. pH of Common Substances

  10. Example—Calculate the pH of a 0.0010 M Ba(OH)2 Solution and Determine if It Is Acidic, Basic, or Neutral. Tro's Introductory Chemistry, Chapter 14

  11. Practice—Calculate the pH of the Following Strong Acid or Base Solutions. • 0.0020 M HCl • 0.0050 M Ca(OH)2 • 0.25 M HNO3 Tro's Introductory Chemistry, Chapter 14

  12. Calculate the Concentration of [H3O+] for a Solution with pH 3.7. Tro's Introductory Chemistry, Chapter 14

  13. pOH • The acidity/basicity of a solution may also be expressed as pOH. • pH = ─log[OH−], [OH−] = 10−pOH • Exponent on 10 with a positive sign. • pOHwater = −log[10−7] = 7. • Need to know the [OH−] concentration to find pOH. • pOH < 7 is acidic; pOH > 7 is basic, pOH = 7 is neutral. Tro's Introductory Chemistry, Chapter 14

  14. pOH, Continued • The lower the pOH, the more basic the solution; the higher the pOH, the more acidic the solution. • 1 pOH unit corresponds to a factor of 10 difference in basicity. • Normal range is 0 to 14. • pOH 0 is [OH−] = 1 M; pOH 14 is [H3O+] = 1 M. • pOH can be negative (very basic) or larger than 14 (very acidic). • pH + pOH = 14.00. Tro's Introductory Chemistry, Chapter 14

  15. Practice—Calculate the pOH and pH of the Following Strong Acid or Base Solutions. • 0.0020 M KOH • 0.0050 M Ca(OH)2 • 0.25 M HNO3 Tro's Introductory Chemistry, Chapter 14

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