K2HPO4 MOLES= 3.9 x0.11= 0.429
addition HCl to K2HPO4 results in a buffer
solution when HCl is added in lesser amount than
K2HPO4
K2HPO4 + HCl ---------------------->
KH2PO4 + KCl
0.429Â Â Â Â Â Â Â Â Â
a                                   Â
0Â Â Â Â Â Â Â Â Â Â Â
0
0.429-a      Â
0Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â
a            Â
a
thus buffer solution contain 'a' moles of KH2PO4 and (0.429-a)
moles K2HPO4
pKa = -log Ka = -log(6.2 x10^-8) = 7.20
For acidic buffer
Henderson-Hasselbalch equation
pH = pKa + log[salt/acid]
7.44 = 7.20 +log[salt/acid]
0.24 = log[(0.429-a)/a]
[(0.429-a)/a] = 10^0.24= 1.737
0.429 -a = 1.737a
0.429= 2.737a
a= 0.157 mole
M x volume = 0.157
4.7 x V= 0.157
V= 0.0334 L
Volume = 33.4ml of HCl must be added
