CHEM 212 Laboratory

CHEM 212 Laboratory Acidity constant from a titration curve Data Analysis Supplement

Reference: General Chemistry Laboratory Experiments, Suzanne W Slayden, 1999, 3rd edition, Pages 165-169

Data: Note down the molarities of standard solutions. During titration note down the volumes and pHs in a tabular form in the laboratory notebook. In your measurements and calculations, remember to pay attention to uncertainties and significant figures.

Results: From the phenolphthalein endpoint, from calculation and from the titration curve, estimate the pH and Veq at equivalence points for HCl and CH3COOH.. Also, calculate Veq /2 (half equivalence point) and pKa of acetic acid. (The lab text refers to Veq as V ml) *There is no download for this experiment.

Your lab report should include the following in addition to the pages from your lab notebook:

 An Excel sheet “Raw Data” showing the volumes of NaOH vs HCl and NaOH vs

CH3COOH. Enter the numbers in the first spreadsheet from your lab notebook.

 A 2nd Excel sheet “Graph-HCl” showing the titration curve of HCl titration (solution pH vs volume of NaOH). On the titration curve, mark the equivalence point and circle it. Draw dotted perpendicular lines to the x and y axes. Estimate the pH and volume of NaOH added at the equivalence point.

rd  A 3 Excel sheet “Graph- CH3COOH” showing the titration curve of CH3COOH titration (solution pH vs volume of NaOH). On the titration curve, mark the equivalence point and circle it. Draw dotted perpendicular lines to the x and y axes. Estimate the pH and volume of NaOH added at the equivalence point. Also calculate the volume

corresponding to half-neutralization (Veq /2). On the graph, draw a dotted line perpendicular to Veq /2 until it intersects the titration curve. Mark and circle the point and then draw a horizontal dotted line which intersects the pH axis. This is the pKa of acetic acid.

Discussion: In your discussion section address the following questions:  Why it is not necessary to use phenolphthalein indicator to detect the endpoint in this titration?

 Can the pKa of acetic acid be calculated from the pH at a volume other than the half- equivalence point? Explain how.

 Calculate the pH at equivalence point using your experimental Ka, concentrations and volumes, and compare it with your experimental pH. Find the % error.  What are the possible sources of error in this experiment?