Acidity Constant by pH Titration Curves

Quiz Lecture Lab!

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

NEW EQUIPMENT: pH Meter

Turn the knob to “pH” Make sure glass bulb is consistently wet

To standardize

1. Rinse bulb briefly with dH2O, blot with paper towel gently

2. Submerge the pH stick in the pH 7 buffer solution solution, be sure to have bulb well in solution

3. Adjust pH knob to the standard solution pH 7.

Procedure Part A Titrate HCl with NaOH

1.Calibrate the pH meter with pH 7 buffer (just use “standardize” knob if necessary)

2.Rinse a graduated cylinder with HCl and add 25 mL HCl to an Erlenmeyer flask

3.Add a few drops of phenolphthalein to HCl

4.Rinse a buret with NaOH, fill with NaOH, and deliver NaOH in increments of 2mL into flask, recording pH

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

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

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

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)

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

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.

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.

Titration Curve

HCl titrated with NaOH (p. 166)

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 = pH @ Volume of Ka Ka = 10^-pKa    

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 

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