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Unformatted text preview: the solution, mix the solution more and determine the pH
again. Continue doing so until the fluctuations in pH are not more than +-0.1. From the pH,
determine the pKa, the Ka, of the acid, and the Gibb’s Free Energy, ∆ Go at equilibrium.
Part II: Influence of temperature
Pour approximately 5 mL of the mixture into two test tubes. Place one into an ice bath, and the
other into a hot water bath. You will want each to reach equilibrium, so go on with at least part
of the rest of the experiment, but do not allow the test tube in the hot water bath to loose too
much volume. Determine the pH of each solution. Determine the heat of equilibrium ∆ Ho .
Part III: Influence of adding reactants or products
Place 5.00 mL of the mixture into four clean, dry test tubes using a pipette. Using the pipettes
provided, put 1.00 mL of the organic acid solution into test tube 1, 1.00 mL of the organic salt
solution, 1.00 mL of 0.100 M HCl into test tube 3 and 1.00 mL of 0.100 M NaOH into test tube
4. Using the methods from class, predict the pH that should be found for each of these
solutions. Using the Pasco system, determine the pH of each of these solutions experimentally.
Recall from class that when [HA(aq)]=[A-(aq)], the resulting solution has pH=pKa, and
the relationship between Gibb’s Free Energy and the equilibrium constant, K=-RTlnK.
Recall van’t Hoff’s equation from class, and the relationship between Gibb’s free energy,
enthalpy and entropy; ∆Go =∆Ho -T∆So ; recall that we need to solve this at 25o C to determine the
change in entropy.
Recall the Henderson-Hasselbach equation from class. Use the value of pKa determined
in part I. Dakota State University page 1 46 of 232 Experiment 12: Le Chatliere’s Principle General Chemistry I and II Lab Manual Observations:
pH of solution:
pKa of solution:
Ka of solution:
∆Go for equilibrium:
Observations: Part II:
pH of cold solution:
Ka at 0o C:
pH of hot solution:
Ka at 100o C:
∆Ho of acid dissociation:
Observations: Dakota State University page 1 47 of 232 Experiment 12: Le Chatliere’s Principle General Chemistry I and II Lab Manual Part III:
Test Tube Additive Predicted
4 Observations: Dakota State University page 1 48 of 232 Experiment 12: Le Chatliere’s Principle General Chemistry I and II Lab Manual Pre-lab Questions:
1. Why are we using an organic acid as the unknown acid?
2. What is special about the mixture that allows us to determine pKa for the acid? Dakota State University page 1 49 of 232 Experiment 12: Le Chatliere’s Principle General Chemistry I and II Lab Manual Post-lab Questions:
1. Now you have the Ka, pKa, ∆Ho and ∆So for the organic acid; any predictions on what it
might be? Explain.
2. How did your predicted values of pH compare with experimental? Can you account for the
3. Suppose you forgot to calibrate you pH probe; all of the readings are off, but you do not know
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