15. For the strong acid-weak base titration of ammonia by hydrochloric acid, find the pH after 15.0 mL of 0.100 M HCl has been added to 25.0 mL of 0.100 M NH3 solution. (Kb = 1.8 x 10-5)

The reaction we will use is:

NH3 (aq) + H+ (aq) NH4+ (aq)

For the stoichiometric calculation:

NH3 (aq) H+ (aq) NH4+ (aq)
INITIAL
0.0025mol
0.0015mol
0mol
CHANGE
-0.0015mol
-0.0015mol
+0.0015mol
FINAL
0.0010mol
0mol
0.0015mol

Converting to concentrations gives us [ NH3 ] = 0.00250 M and [ NH4+ ] is 0.0375 M.

Next, we want to find out what the [ OH- ] will be when these two species are at equilibrium with each other. ( We will find [ OH- ] and not [ H+ ] because we only know Kb ).

An alternate method

The equation for this part is NH3 (aq) + H2O (l) NH4+ (aq) + OH- (aq)

The equilibrium constant expression will be :

a) Kb = {[ OH- ](0.0375)} / {0.025}
b) Kb = {[ OH- ](0.025)} / {0.0375}
c) Kb = {[ OH- ]} / {(0.025)(0.0375)}
d) Kb = {(0.0375)(0.025)} / {[ OH- ]}