Experiment 9 Weak Acid and Buffers.docx - CH 204 u2013...

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CH 204 – Introduction to Chemical Practice Experiment #9 – Weak Acids and Buffers Mia Trinh Lauren Enten TA: Atreya Dey April 8, 2018 INTRODUCTION Experiment 9 had the purpose of titrating acetic acid with sodium hydroxide (NaOH) and then a buffered solution of acetic acid with NaOH and hydrochloric acid (HCl). In Part A, the acetic acid was titrated with the sodium hydroxide in order to find the equivalence point and the half-equivalence point of the titration. The equivalence point was found by first creating a second derivative curve from the original titration curve for each trial. Then once a second derivative curve was made, the equivalence point was where the curve crossed the y-axis. This volume represents when the moles of the acid equal the moles of the base in solution. 1 The half- equivalence point was found by dividing the equivalence point volume by two and finding the pH value at that volume on the original titration curves. The pH at the half-equivalence points were then used to find the acid dissociation constant, pK a , which equaled the pH. This can be shown through the Henderson-Hasselbalch equation, pH = pK a + log([A - ]/[HA]). 2 In the case of titrating a weak acid with a strong base, the half equivalence point represents when the moles of each equal each other, canceling out the log portion of the equation. pH then equals pK a and can be used to find the K a of the acid. In Part B, the buffered acetic acid solution was titrated with NaOH and in Part C, it was titrated with HCl. The solutions were titrated with the strong acid and base by a burette. A burette was used in order to give accurate measurements for each increment added to the solution. 3 The buffered solution was titrated until the buffer capacity was reached. The buffer capacity was reached when the pH was changed by 1 pH unit at a certain volume. These values were recorded in a table and represented by the following graphs. Once the measured pH and the theoretical pH were found, the percent error was able to be calculated through the equation, % error = |(theoretical pH - measured pH)/theoretical pH| x 100.

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