In: Chemistry
Consider the titration of 60.0 mL of 0.100 M NH3 Kb= 1.8 x 10^-5 with 0.150 M HCl. Calculate the pH after the following volumes of titrant have been added: 40.5 mL & 60.0 mL
mmoles of NH3 = 60.0 x 0.100 = 6.00
Kb = 1.8 x 10^-5
pKb = 4.74 , pKa = 9.26
a)
mmoles of HCl = 40.5 x 0.150 = 6.075
NH3 + HCl ------------> NH4+Cl-
6.00 6.075 0
0 0.075 6.00
here strong acid remains. so
concentration of H+ = 0.075 / 60 + 40.5
= 7.46 x 10^-4 M
pH = - log (7.46 x 10^-4)
pH = 3.13
b)
mmoles of HCl = 60 x 0.150 = 9
NH3 + HCl ------------> NH4+Cl-
6.00 9.00 0
0 3.00 6.00
here strong acid remains. so
[H+] = 3 / 120 = 0.025 M
pH = 1.60