Question

Sort the pH of the following mixtures from LOWEST (most acidic) to HIGHEST (most basic):

(I) 0.001 mol of HCl added to 1 L water

(II) 0.001 mol HCl added to a buffer solution of 0.1 M HCOOH and 0.1 M NaHCOO (pKa=3.75)

(III) 0.001 mol HCl added to a buffer solution of 0.1 M NH4Cl and NH3 (pKa=9.25)

Answer 1

1) concentration of HCl = n/v = 0.001/1 = 0.001 M

   pH = -log(H3O+)

      = -log0.001

   = 3

2) pH of acidic buffer = pka+log(NaHCOO-HCl/HCOOH+HCl)

           pka of HCOOH = 3.75

pH = 3.75+log((0.1-0.001)/(0.1+0.001))

     = 3.74

3) pH of basic buffer =14 - ( pkb+log(NH4Cl+HCl/NH3-HCl))

pkb of NH3 = 14-9.25 = 4.75

      = 14 - (4.75+log((0.1+0.001)/(0.1-0.001))

      = 9.24

order of pH : III > II > I