Acids, Bases, and Buffers 1: Monoprotic and Polyprotic Acids Overview Getting Started Techniques Procedure FAQ Full Lab Manual Introduction & Goals Chemistry & Background Key Questions Prelab Problems Safety Procedure In Your Write-up Experiments Index ChemLab Home		Before Lab Before coming to lab this week, complete the five prelab problems in your manual. Complete your prelab preparation in your lab notebook, which includes the objective, reference, prelab procedure, and sample calculations. Make sure that your procedure and sample calculations or analysis flowcharts are detailed and complete. In Lab Record your observations and any changes from your prelab procedure. Plot full-page experimental titration curves, as you measure them. Cite your lab partner's notebook, if the original data appears there. On your plots, label the values of equivalence and half-equivalence points. Record the buffer solution pH and acidic and basic buffer capacity. Plot the acidic and basic buffer capacities on a graph similar to Figure 2 in the introduction of the experiment in the manual. Uncertainty Analysis For your uncertainty analysis, include the following: • While in the lab, estimate and record the uncertainty in buret readings and the uncertainty in pH readings. These are the two contributions to the uncertainty in your experimental pKa values. • On your phosphoric acid titration curve, draw error bars showing the uncertainty in pH and in titrant volume at the half-equivalence point. Make a rectangle around the first half-equivalence point showing the uncertainty in pH and volume. You should replot the area around the half-equivalence point with a larger scale, to show the error accurately. • Use this "error rectangle" to estimate the uncertainty in your experimental, graphically determined pKa value. Which data measurement, pH or volume, contributes the most to the uncertainty of your results? After Lab Your results should include • A calculation of the molarity of acetic acid in vinegar. • An experimental titration curve for the titration of vinegar with strong base, with the equivalence and half-equivalence points clearly labeled. Determine the pKa of the acid from this graph. • The composition and pH of your acetic acid/acetate buffer solution. Report the acidic and basic buffer capacities and plot both titrations on the same axes, as in Figure 2 from the introduction of the experiment in the manual. • Experimental titration curve of pH vs. milliliters NaOH added for phosphoric acid. On your plot, label the equivalence points and half-equivalence points, for each dissociation. Also include your graphical uncertainty analysis, as described above. • Calculation of the three experimental pKa values of phosphoric acid from the half-equivalence point, 3/2 equivalence point, and 5/2 equivalence point. Note that your experimental values will not agree precisely with the literature values given in the manual, which are theoretical values for infinite dilution of the species.
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