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Determine Ka of a Weak Acid

Determine Ka of a Weak Acid. Determine Ka of a weak acid. The ionization of a weak acid can be shown by the following equation: HA (aq) + H 2 O (l)  H 3 O + (aq) + A - (aq) (Eqn. 1) Since an equilibrium exists, an equilibrium constant, Ka, can be written as:

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Determine Ka of a Weak Acid

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  1. Determine Ka of a Weak Acid

  2. Determine Ka of a weak acid The ionization of a weak acid can be shown by the following equation: HA(aq) + H2O (l)  H3O+(aq) + A-(aq) (Eqn. 1) Since an equilibrium exists, an equilibrium constant, Ka, can be written as: Ka = [H3O +] [A-] (Eqn. 2) [HA] Rearrange eqn (2) and take –log on both sides, pH = pKa – log ([HA]/[A-]) (Eqn 3)

  3. pH = pKa – log ([HA]/[A-]) • At a point halfway to the equivalence point, i.e. the acid is half-neutralized, • [HA] can be assumed to be equal to [A-] for a monobasic acid. pH = pKa – log ([HA]/[A-]) pH = pKa – log (1) pH = pKa • Simply measure the pH of the half-neutralized acid mixture, pKa of the weak acid can be found.

  4. Determine Ka of a weak acid Two methods can be used to determine the Ka value. Both methods require the use of a pH meter. The first method is to find the Ka from a titration curve. i.e. a sample of weak acid is titrated with a base, the pH values are plotted vs. the volume of base added.

  5. Method 1: To find the Ka of a weak acid from the titration curve ?????? How??? Where?? Titration curve for the titration of 0.1M CH3COOH with 0.1M NaOH

  6. Task 1: Your task is to design an experiment (including apparatus set-up) to determine the Ka value for ethanoic acid by means of a titration curve. Each group have to draw the apparatus set-up and write down how to determine the Ka from the titration curve. You may refer to the equipment and material provided in the expt. manual.

  7. Apparatus used for getting the titration curve

  8. Method 1: By using the titration curve Equivalence Point

  9. Determine Ka of a weak acidMethod 2: By using the “half volume” method To determine the pH of the solution mixture when half of the weak acid has been neutralized.

  10. How can I know that the acid is half-neutralized ???

  11. Task 2: Your task is to design an experiment (including apparatus set-up) to determine the Ka value for ethanoic acid using the “half volume” method. You may refer to the equipment and material provided in the expt. manual.

  12. Apparatus used for getting the acid half- neutralized

  13. A solution of the acid is prepared and divided in half as accurately as possible. One portion (25.0ml) is titrated to its end point with phenolphthalein. The two portions are then recombined, and the pH of the resulting solution is measured. Since half of the acid (25.0ml) has been titrated, therefore [HA] is assumed to be equal to [A-]. Method 2: By using the “half volume” method

  14. Standardization of pH meter Procedures: 1) Set the temperature knob to the temperature of solution. 2) Place the glass electrode into a beaker of distilled water for a while. 3) Then put it into a suitable buffer solution of known pH value. 4) Adjust the standardization knob until it reads the pH values of buffer solution. Put the standardized pH meter into a beaker of distilled water when not in used. 5) When a solution with unknown pH is to be measured, put the glass electrode into the beaker of unknown solution, when the reading becomes steady, measure the pH value. 6) After taking the reading, rinse the glass electrode with a little distilled water from a washing bottle and then place it in a beaker of clean distilled water. 7) After finish taking all the readings, the pH meter should be washed thoroughly with distilled water and then put back to the bottle of storage solution.

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