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Buffer solutions

Buffer solutions. Lab 9. introduction. Adding trace amount of acid to water will result in increase in H + concentration. In the same manner, adding trace amount of alkali eg . NaOH to pure water will result in increase of OH −

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Buffer solutions

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  1. Buffer solutions Lab 9

  2. introduction • Adding trace amount of acid to water will result in increase in H+ concentration. In the same manner, adding trace amount of alkali eg. NaOH to pure water will result in increase of OH− • So we can say that water has no ability to resist change in H+ concentration or pH. (pH = − log [H+])

  3. What is a buffer solution? • Buffer solution: • Is a system that possesses the property of resisting change in the pH with the addition of small amount of strong acid or base. • The efficiency of the system called buffer capacity.

  4. Composition • Buffer solution is usually composed of a weak acid and a salt of the acid eg. Acetic acid and sodium acetate, or weak base and a salt of the base eg. Ammonium hydroxide and ammonium chloride.

  5. Weak acid buffer • Weak acids do not completely dissociate when in solution, instead an equilibrium is set up;HA ⇌ H+ + A− • The dissociation constant or value is calculated by the following equation: • Where = salt • = acid

  6. Weak acid buffer • Dissociation constants of some weak acids at 25 °C

  7. Weak acid buffer • …… (1) • Since most values are small numbers its more convenient to express them as negative logarithm • p = − log • So equation (1) will be • and because pH = • Then= pH− • andpH = + • Weak acid buffer pH is ˂ 7

  8. Weak base buffer • BOH ⇌ B++ OH− • Similary, the dissociation constant, or Kbvalue of weak base is given by the equation: • Where B+ = salt • and = base • pH = − p Kb+ • = 14

  9. Practice problems • the dissociation constant Kaof acetic acid is 1.75 x 10-5at 25 °C. Calculate its pKavalue. • What is the pH of a buffer solution prepared with 0.05 M sodium borate and 0.005 M boric acid? The pKaof boric acid is 9.24 at 25 °C. • What is the pH of a buffer solution prepared with 0.05 M ammonia and 0.05 M ammonium chloride? The Kb of ammonia is 1.8 x 10-5 at 25 °C. • What molar ratio of salt/acid is required to prepare a sodium acetate-acetic acid buffer solution with a pH of 5.76? pKaof acetic acid is 4.76 at 25 °C.

  10. Practice problems • The molar ratio of sodium acetate to acetic acid in a buffer solution with a pH of 5.76 is 10:1. Assuming the total buffer concentration is 2.2 x 10-5mol/L, how many grams of sodium acetate (m.w. 82) and how many grams of acetic acid (m.w. 60) should be used in preparing a liter of the solution? • Calculate the change in pH after adding 0.04 mol of sodium hydroxide to a liter of buffer solution containing 0.2 M concentration of sodium acetate and acetic acid. The pKaof acetic acid is 4.76 at 25 °C. • What is the pH of buffer solution prepared with 0.055 M sodium acetate and 0.01 M acetic acid? The pKa of acetic acid is 4.76 at 25 °C.

  11. Home work • What is the change in pH with addition of 0.01 hydrochloric acid to a liter of buffer solution containing 0.05 M of ammonia and 0.05 M ammonium chloride? Kb of ammonia is 1.8 x 10-5 at 25 °C. • The dissociation constant of ethanolamine is 2.77 x 10-5 at 25 °C. Calculate pKb

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