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Acidity Constant by pH Titration Curves

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1 Acidity Constant by pH Titration Curves

2 Quiz Lecture Lab!

3 Titrations: Endpoints
Three ways to determine endpoint: Perform a calculation using M1V1=M2V2 Calculation – Theoretical Using the visual indicator (phenolphthalein) Experimentally Use a titration curve Experimental with calculations You will be comparing the accuracy of visual indicator vs. pH curve to the theoretical value

Turn the knob to “pH” Make sure glass bulb is consistently wet To standardize Rinse bulb briefly with dH2O, blot with paper towel gently Submerge the pH stick in the pH 7 buffer solution solution, be sure to have bulb well in solution Adjust pH knob to the standard solution pH 7.

5 Procedure 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 flask Add a few drops of phenolphthalein to HCl Rinse a buret with NaOH, fill with NaOH, and deliver NaOH in increments of 2mL into flask, recording pH

6 Remember 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

7 Procedure 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

8 WARNING!!! 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

9 End 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 cup Check with lab prep to clean up Turn in yellow copies (pre-lab and filled in data tables and observations)

10 Due 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 

11 Formal Lab Report Title Page Data Table (excel)
NaOH + HCl raw data pH vs. volume NaOH NaOH + Acetic Acid raw data pH vs. volume NaOH added Bold the volume amount of NaOH in which you got a pink endpoint.

12 Graphs 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.

13 HCl titrated with NaOH (p. 166)
Titration Curve HCl titrated with NaOH (p. 166)

14 Calculations – graph Sample calculations for BOTH HCl and Acetic Acid – phenolpthalein, theoretical, graph Titration curve -- graph Justify how you found the equivalence point, the volume of equivalence point, and its correlating pH. Find Ka     Remember pKa = Volume of Ka Ka = 10^-pKa    

15 Calculations Sample calculations for BOTH HCl and Acetic Acid – phenolpthalein, theoretical, graph  Phenolphthalein endpoint – find volume equiv point Get volume final volume - initial volume of NaOH added Theoretical endpoint – find volume equiv point use Macid*Vacid=Mbase*Vbase 

16 Conclusions 1 page -- refer to lab report outline for general questions and how to do in depth analysis Analyze graph, why you use it? Compare Ka of acetic acid versus HCl Compare Vol Equivalence pt of pH meter vs. phenolphthalein, which is more accurate? Error analysis -- what happened experimentally that can affect the values

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