Question

Calculate the pH of a buffer solution that is obtained when 1.727 mL of 3.0 M HCl are added to a 48.273 mL of aqueous solution in which 0.727 g of Tris base has been dissolved. The allowed error is 0.02 pH units.

Answer 1

no of mole of tris base = W/G.M.Wt

                                     = 0.727/121 = 0.006moles

no of moles of HCl   = molarity* volume in L

                               = 3*0.001727= 0.005181moles

                 tris base + HCl --------> tris salt

I                0.006        0.00518           0

C            -0.00518     -0.00518         0.00518

E              0.00082        0                 0.00518

     POH = pkb +log[tris salt]/[tris base]

             = 5.92 + log0.00518/0.00082

             = 5.92 + 0.8 = 6.72

PH    = 14-POH

           = 14-6.72 = 7.28 >>answer