workshop 5: Titration curves A pH or titration curve can be produced by plotting
ID: 488759 • Letter: W
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workshop 5: Titration curves A pH or titration curve can be produced by plotting the pH of a solution (containing an analyte) versus the volume of added titrant. The equivalence or end point of a titration occurs when enough titrant has been added to consume all the acid or base initially present in the analyte. You know you have reached the endpoint when you see a significant change in the pH of the solution. Part 1 Let's begin by analyzing a titration curve when your analyte is a weak acid or base the titration curve shows the effects of buffering before the equivalence point. The midpoint of the titration occurs in the buffer zone and is the point at which the pH is equal to the pKa of the analyte. In the case of a weak acid-strong base titration the pH at the equivalence point will always be higher than 7. This is because of the basic properties of the anion (the conjugate base of the weak acid) that is present in the solution at this point. To the right is a titration curve for the Titration of unknown acid titration of 25.0 mL of unknown. compound X with o 200 M NaOH. Use this 14 titration curve to answer the following 12 questions. 10 (a) what is the identity of compound X? pH HINT: Use the Chempendix on Sapling to look at the Ka values for possible weak acids. 50 20 300 400 10 volume of o.200 M NaOH added, mL (b) what is the starting concentration of compound X?Explanation / Answer
For the given titration curve
(a) unknown compound X = benzoic acid
(b) Table
NaOH (ml) major species in solution pH active species pH
0 C6H5COOH C6H5COOH 2.45
30 C6H5COO- C6H5COO- 8.62
30.75 C6H5COO- OH- 11.43
40 C6H5COO- OH- 12.50
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