3Titrations: Endpoints Three ways to determine endpoint:Perform a calculation using M1V1=M2V2Calculation – TheoreticalUsing the visual indicator (phenolphthalein)ExperimentallyUse a titration curveExperimental with calculationsYou will be comparing the accuracy of visual indicator vs. pH curve to the theoretical value
4NEW EQUIPMENT: pH Meter Turn the knob to “pH”Make sure glass bulb is consistently wetTo standardizeRinse bulb briefly with dH2O, blot with paper towel gentlySubmerge the pH stick in the pH 7 buffer solution solution, be sure to have bulb well in solutionAdjust pH knob to the standard solution pH 7.
5Procedure Part A Titrate HCl with NaOH Calibrate the pH meter with pH 7 buffer (just use “standardize” knob if necessary)Rinse a graduated cylinder with HCl and add 25 mL HCl to an Erlenmeyer flaskAdd a few drops of phenolphthalein to HClRinse a buret with NaOH, fill with NaOH, and deliver NaOH in increments of 2mL into flask, recording pH
6Remember you are noting the pH endpoint AND visual endpoint At ~20 mL decrease to ~0.4 mL increments (the equivalence point is around here!!)Note the volume in which you reached visual endpoint (pink)Record pH until pH is 10, then increase increments to 2 mL till pH probe reads consistent numbers
7Procedure Part B Titrate CH3COOH with NaOH Same procedure but with 25 mL acetic acid + indicator in Erlenmeyer flask!Titrate with the NaOH from the buret
8WARNING!!! DO NOT use a beaker for this experiment! DO NOT stir solution with pH probe!Add NaOH, gently remove buret from flask, swirl solution, insert pH probe, record value.For best results, record buret to the nearest 0.0x mL
9End of Lab Pour solutions down the sink Clean your lab station Turn off pH meter, make sure to keep pH stick and glass bulb in the buffer solution cupCheck with lab prep to clean upTurn in yellow copies (pre-lab and filled in data tables and observations)
10Due in TWO weeks Next week you have spring break…which means no lab But in two weeks a formal lab is due ….that and the pre-lab/ quiz for Expt 8. Molar Solubility and the Common-Ion Effect
11Formal Lab Report Title Page Data Table (excel) NaOH + HCl raw data pH vs. volume NaOHNaOH + Acetic Acid raw data pH vs. volume NaOH addedBold the volume amount of NaOH in which you got a pink endpoint.
12Graphs 2 graphs: one for HCl and one for Acetic Acid Make a calibration curve pH (Y-axis) vs. Volume NaOH (mL)Mark the pH equivalence point, the phenolphthalein equivalence point, and the half-neutralization point/pKA for each graph.
13HCl titrated with NaOH (p. 166) Titration CurveHCl titrated with NaOH (p. 166)
14Calculations – graphSample calculations for BOTH HCl and Acetic Acid – phenolpthalein, theoretical, graphTitration curve -- graphJustify how you found the equivalence point, the volume of equivalence point, and its correlating pH.Find Ka Remember pKa = Volume of KaKa = 10^-pKa
15CalculationsSample calculations for BOTH HCl and Acetic Acid – phenolpthalein, theoretical, graph Phenolphthalein endpoint – find volume equiv pointGet volume final volume - initial volume of NaOH addedTheoretical endpoint – find volume equiv pointuse Macid*Vacid=Mbase*Vbase
16Conclusions1 page -- refer to lab report outline for general questions and how to do in depth analysisAnalyze graph, why you use it?Compare Ka of acetic acid versus HClCompare Vol Equivalence pt of pH meter vs. phenolphthalein, which is more accurate?Error analysis -- what happened experimentally that can affect the values