Question

A buffer solution contains 0.257 M NH₄Br and 0.482 M NH₃ (ammonia). Determine the pH change when 0.124 mol HBr is added to 1.00 L of the buffer.

pH after addition − pH before addition = pH change = ?

Answer 1

First,

the buffer equation

pH = pKA + log(NH3/NH4+)

pH = 9.25 + log(NH3/NH4+)

initial

mol of NH3 = MV = 0.482*1 = 0.482 mol

mol of NH4+ = MV = 0.257 ! = 0.257 mol

initial pH:

pH = 9.25 + log(NH3/NH4+)

pH = 9.25 + log(0.482 /0.257 )

pH = 9.523

now...

after HBr is added, H+

H+ reacts with NH3 to form NH4+

mol of NH3 = 0.482 -0.124 = 0.358

mol of NH4+ = 0.257 +0.124 = 0.381

substitute

pH = 9.25 + log(NH3/NH4+)

pH = 9.25 + log(0.358 /0.381)

pH = 9.222

change in pH =9.222-9.523 = 0.301(decrease)