9. When the pH reading reaches 9 you can add 1 to 2 mL portions of NaOH again 10. When you reach a pH of around 11 you are done. 11. Press the stop button on the LabQuest and export the data to a thumb drive. 12. Use the obtained data to calculate the concentration of the titrated acid (HCl). 13. Use the calculated molarity value in experiment IV. Procedure for titrating a Weak Acid 1. Place 25 mL ( via pipet ) of Acetic acid solution in a clean labeled 100 mL beaker. Add a stirring bar and a drop of phenolphthalein. ( The nominal molarity of the acetic acid is the same as the HCl. ) 0.1 M Color change pH: 9.2 2. Repeat steps 2-12 above to titrate the acetic acid solution. Disposal: Place all solutions in the properly labeled waste bottle in the waste hood. Conclusion In conclusion, the equivalence point was determined by measuring the pH of a solution after titrating with a weak acid and a strong acid. After graphing the points, the equivalence point was confirmed. In the first trial, we titrated with 25 mL of the strong acid, HCl, with 26 mL of .1M NaOH. In the second trial, we titrated with 25 mL of the weak acid, Acetic Acid, with 21 mL of .1M NaOH. In the first experiment the equivalence point was at 23.4 mL; thus, the half equivalence was at 11.7 mL. We observed a color change at a pH of 8.2. In the second trial, the equivalence point was at 18.7 mL, meaning the half-equivalence occurred at 9.4 mL. We observed a color change at a pH of 9.2. It was very important to slowly add the NaOH, because by adding too much at a time a smooth curve could c831c73664064c18a091396a7983016aeaa3506c.docx 3
not be produced, neither could a steady color change. By adding too much NaOH, the color of the solution immediately turns into a hot pink, as opposed to a gradual light to dark pink increase. Graphs 0 5 1 3 5 7 9 11 13 HCl and NaOH Titration Curve NaOH Added (mL) pH 0 5 10 15 20 2 4 6 8 10 12 14 AcOH and NaOH Titration Curve NaOH Added (mL) pH c831c73664064c18a091396a7983016aeaa3506c.docx Equivalence Point: 23.4 mL mLmLmL23.2323.7mL Half Equivalence: 11.7 mL
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