posted by Keith .
A solution of an unknown monoprotic weak acid was titrated with 0.100 M NaOH. The equivalence point was reached when 37.48 ML of base had been added. From a second buret, exactly 18.74 of 0.100 M HCl were added to the titration solution. The pH was then measured with a pH meter and found to be 4.13. Calculate the acidity constant of the weak acid.
When a weak acid is titrated half way to its equivalence point(18.74 is 1/2 of 37.48), the pH at that point = pKa.
Solve for Ka from pKa = -log Ka