Dissociation Constant Weak Acids

Introduction: The dissociation constants of weak monoprotic acids were determined with the aid of a buffer and pH meter.

Experiment: This experiment was performed according to the procedures as provided in the Chemistry 111B laboratory manual: Po, Henry and Senozan, N. "General Chemistry Experiments Chemistry 111B", 8th Ed., 2004 Hayden-McNeil Publishing, pp 80-88. To determine the dissociation constants of weak monoprotic acids (HX and HY), a buffer was needed to calibrate the pH meter. According to the manual, all four solutions to the experiments had to be carried out first before the calibration was performed.

The following materials were obtained to conduct the parts of the experiment: 1) several 10-mL and 50-mL graduated cylinders to measure the volumes of acid and base, and 2) a thermometer to record the temperature of the solution.

In the first half of the experiment, the Ka of HX had to be used and determined and in the second half, the Ka of HY was utilized and established using the same procedure.

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In the first part of the experiment, the pH and temperature was to be measured of a 10 mL sample of 1.02 M HA (when conducting the second half of the experiment, the concentration of HY was 1.00 M). The second part of the experiment was to add 20 mL of 1.0 M NaA to 10 mL of 1.00 M HA, and to draw 10 to 15 mL of the solution to measure the pH and temperature. For the third part of the experiment, 10 mL of the remaining buffer from part 2 was diluted with 50 mL of distilled water.

The pH and temperature was measured for this part of the experiment. The last part of the experiment was to mix 10 mL of NaOH to 25 ml of HA (both HX and HY had a concentration of 1.00 M, and the NaOH concentration was 1.00 M), and to measure the pH and temperature of the solution.

When all the solutions were stirred and measured for, the pH meter was to be calibrated. The first step was to obtain a packet of perpHect pH 4.01 buffer and set up the open packet in a beaker. The next step was to place the pH meter into the buffer and calibrate the reader that was connected to the meter. When the reader was calibrated to a pH of 4.01, the solutions were ready to be determined. After each measurement in the solution, the pH meter had to be cleaned with a few drops of distilled water and calibrated back to 4.01 in the buffer pouch.

Results and Discussions: In the first half of the experiment using HX, the pH reading for the first part of the experiment was 2.36 and the temperature was 26 C. When equilibrium was reached, it was calculated (on data sheet) that the Ka was 2.03 X 10 .In the second part of the experiment, the pH was 4.94 and the temperature was 28 C. The Ka was found to be 2.07 X 10 . For the third part of the experiment, a 4.47 pH was measured from the solution and a temperature of 27 C. The Ka for this part was 2.57 X 10 . Lastly, the pH of the solution for part 4 was 4.47 and had a temperature of 27 C with a Ka reading of 2.29 X 10 .

For the second part of the experiment dealing with HY, readings of 1.95, 25 C, and 1.27 X 10 were measured for pH, temperature, and Ka, respectively. In the second part of the experiment, the pH reading was 2.94 with a temperature of 26 C and a Ka of 2.31 X 10 .In the third part of the experiment, a pH measurement, temperature, and Ka values were found to be 2.95, 26 C, and 2.32 X 10 , respectively. In the last part of the experiment, the pH reading was 2.58 with a temperature of 27 C. The Ka for this particular solution was 1.78 X 10 .

Assumptions that were made from this experiment was that the Ka values for HX and HY were similar except for part 1. Part 1 was excluded in the averages in the data sheet because since no base is present in the solution, a different Ka value will be given then the other solutions which had base in the solution. Also, since the solutions of the experiment were prepared first before calibrating the pH meter, the group received a better pH reading because the pH of the solutions were measured one after the other.

Conclusion: In the experiment, it was seen that the dissociation of weak monoprotic acids were constant for HX and for HY given from the Ka values.