pka of phosphate buffer = 7.2
no of mole sodium dihydrogen phosphate =
M*V
                                                                        Â
= 87.8*0.0264
                                                                       Â
= 2.32 mmole
no of mole sodium hydrogen phosphate
(Na2HPO4) = ?
pH of acidcbuffer = pka + log(salt/acid)
                        Â
7.78 Â Â Â Â Â = 7.2 +
log(x/2.32)
            Â
x = no of mol of (Na2HPO4)Â Â = 8.82
mmole
  amount of sodium hydrogen phosphate
(Na2HPO4) = n*Mwt
                        Â
= 8.82*10^-3*142
                         Â
= 1.25 g