Question

9) You wish to prepare a buffer which has a pH of 4.9. Choosing among benzoic acid/benzoate (Ka for benzoic acid is 6.28 × 10−5), chlorous acid/chlorite (Ka for chlorous acid is 1.12 × 10−2), and hypobromous acid/hypobromite (Ka for hypobromous acid is 2.3 × 10−9), and assuming you want to prepare 1 L of this buffer and you wish to have the buffer 0.25

M

with respect to the acid, what mass of the sodium salt of its conjugate base will be necessary to produce the buffer at the desired pH? (You still might have to adjust the solution to get the right pH, but do this computation as a ballpark estimate of what's necessary!) Report your answer to 3 significant figures.

Answer 1

pka of benzoic acid = -logka

                    = -log(6.28 *10^-5)

                    = 4.2

pka of chlorous acid = -logka

                    = -log(1.12 *10^-2)

                    = 1.95

pka of hypobromous acid = -logka

                    = -log(2.3 *10^-9)

                    = 8.64

so that, best one to get the desire buffer is benzoicacid

pH of acidic buffer = pka +log(C6H5COO-/C6H5COOH)

        pka = 4.2

total no of mol of buffer = 0.25*1 = 0.25 mol

   4.9 = 4.2 +log(x/(0.25-x))

x = 0.21

No of mol of C6H5COONa = x = 0.21 mol

amount of C6H5COONa = n*Mwt

                     = 0.21*144.12

                     = 30.3 g

No of mol of C6H5COOH = 0.25-0.21 = 0.04 mol

amount of C6H5COOH = n*Mwt

                   = 0.04*122.12

                   = 4.90 g