Question

Addition of a small amount of an aqueous solution of sodium hydroxide to an aqueous solution of ammonia will (please give explanation)

A.increase both the pH and Kb of ammonia.
B.increase the pH but will not affect Kb of ammonia.
C.decrease both the pH and Kb of ammonia.
D.decrease the pH but will not affect Kb of ammonia.
E.decrease the pH but will increase Kb of ammonia.

Answer 1

answer : D.decrease the pH but will not affect K_b of ammonia.

NH4OH ------------------------> NH4+ + OH-

NaOH---------------------------> Na+    + OH-

due to common ion OH- the degree of dissociation of ammonia solution decreases. so OH- ion concentration decrease . pOH value increases so pH decreases

POH = -log[OH-]

[OH-] decreases pOH increases

pH + pOH = 14

pH = 14- pOH

pOH increases pH decreases

NH3 is weak base . on addition of strong base NaOH , the pH of resulting solution will be decreased. because strong base NaOH .

Kb of NH3 = 1.8 x 10^-5. the ammonia Kb value doesn't effect by strong base or strong acid . it is equilibrium constant.equilibrium constant value only depends on temperature . so Kb value depends on temperature.