A 125.0 mg sample of an unknown, monoprotic acid was dissolved in 100.0 mL of distilled water and

Question

titrated with a 0.050 M solution of NaOH. The pH of the solution was monitored throughout the titration,
and the following data were collected. Determine the Ka of the acid.
Volume of
OH- (mL) 0 5 10 15 17 18 19 20 21 22 22.2 22.6 22.8 23 24 25 30 40
pH 3.09 3.65 4.10 4.50 4.55 4.71 4.94 5.11 5.37 5.93 6.24 9.91 10.2 10.4 10.8 11.0 11.5 11.8

Answer 1

Plot the data as pH on the y axis and mL on the x axis. Locate the equivalence point, read the mL, divide that by 2 and read the pH at that mL mark (the mL/2 mark that is) and that will be the pKa value for the weak acid.