Experiment 25 Determination of K a for a Weak Acid CHE1181

In Lab Work in groups of 2 NaOH is already prepared; your lab instructor will give you the molarity of the NaOH Do parts C and D: part C is the experiment, part D is only calculations CHE1182

Part C Make sure the starting volume in the buret is exactly 0.00 mL Buret volume readings should have 2 digits after the decimal point Use the pH meter to determine the pH of the solution in flask after each addition of titrant. CHE1183

Part D Calculations only, no lab work. Use the data from part C. CHE1184

Calculations Part C For each trial: 1.Make a titration curve 2.Determine the equivalence volume 3.Determine the (½ equivalence volume) 4.Determine the pH at the ½ equivalence volume 5.pK a = pH at the ½ equivalence volume 6.K a = 10 –pKa CHE1185

Calculations Part D CHE1186 For each trial: 1.The volume of acid is the volume put into the Erlenmeyer flask in part C. 2.The molarity of NaOH is given by lab instructor. 3.The mL of NaOH at equivalence pt. is from curve. 4.M of unknown acid = moles acid / L of acid 5.Moles acid = moles base = (M NaOH ) (L of base)