Experiment 25

Determination of Ka for a Weak Acid

CHE118 1

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

CHE118 2

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.

CHE118 3

Part D

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

CHE118 4

Calculations Part C

For each trial:1.Make a titration curve 2.Determine the equivalence volume3.Determine the (½ equivalence volume)4.Determine the pH at the ½ equivalence volume5.pKa = pH at the ½ equivalence volume

6.Ka = 10 –pKa

CHE118 5

Calculations Part D

CHE118 6

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 acid5.Moles acid = moles base = (MNaOH) (L of base)