EXPERIMENT 4: Use this procedure for plotting the first derivative curve in Excel instead of the one in the lab manual. 10 bonus points

                   Using Excel Spread Sheet  

          Make sure you tabulate your data as shown below. Hand in this tabulated data with your report.

            A                           B                              C                      D                       E                       F

Notes for Experiment 4

 For cell values used above:

Your first plot of pH vs Volume of NaOH should give you a 'visual' indication of where the equivalence point is and therefore the half-equilvalence point. The second plot of the 1st derivative of pH with respect to volume vs the aveage volume of NaOH titrated should give you an inflection point that pinpoints the equivalence point more accurately.

Plot the first derivative (dpH/d vol) vs the average volume of 0.1m NaOH titrated (see highlighted paragraph below) and look for spike in graph which is the point where the first derivative is a maximum. This should coincide with your equivalence point that you found by visual inspection of the titration curve.  If you found an equation for this plot using a 3^rd or 4^th order polynomial on Excel, you can differentiate this equation which will be the 2^nd derivative and set it equal to zero and solve for x. This should be close to the volume you found from the first derivative plot (volume where ‘spike’ occurs’).

You do NOT need to plot a second derivative graph.  Just plot the first derivative data in column E vs the average volume in column F. If you like, you can use Excel to find a 3^rd order polynomial equation to fit this data and then differentiate it mathematically. Set it equal to zero and solve for x.

 Remember that half this volume is where the half-equivalence point occurs and that is where the pH = pKa of the weak acid that you titrated. Study the section on the Henderson-Hasselbach equation on pg 689, 695 which relates to finding the pKa in this experiment. 

You can also check the sample graphs I made using excel for comparison, My pH values are made-up so don’t compare numerical results just the approach I used.

 In your conclusion, show the Pka and the Ka found as well as the name of the unknown acid, and the molecular weight (if solid unknown was used) or concentration (m/L) if a liquid unknown was used. Explain possible sources of experimental error and how your value compares with published value

 Check this link for pKa values of weak acids.  From your results, which acid did you have? 

SUPPLEMENTARY QUESTIONS:

Answer these 4 questions instead of the ones on the lab manual. The are basically the same with some hints added.

 For the 4 supplemental questions in the lab, write intelligent answers using your text book as a guide and the internet as another source of answers

 Question1. Same as manual. No change to question. Answer by examining graph and using the equilibrium equation
                       for finding pH = -Log [H⁺]

 .Question 2: Same as Lab Manual: For example the Henderson-Hasselbach equation is important for determining the
                         Pka (question 2). Explain how or why. Use equation(s) and good sentences to explain.

 Question 3 should be obvious from your 2 experimental plots.  The second derivative plot of the original data would
                       yield the same answer as taking the derivative of the polynominal equation derived from the first derivative
                       plot and solving for x when you set that equation to zero. Explain why all 3 methods should yield the
                       same answer for the equivalence point.

 Question 4.  Explain what happens when diprotic acids (2 protons per molecule of acid) are titrated. (See  text on
                         pp 650-655). Show in a diagram below is what the titration curve would look like. See the links below for
                         additional  references.  Link 3 below is very good visually

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                                                                        Page last updated on 10/17/2007