A buffer solution contains 0217 M ammonium bromide and 0440

A buffer solution contains 0.217 M ammonium bromide and 0.440 M ammonia. If 0.0538 moles of hydrobromic acid are added to 250 mL of this buffer, what is the pH of the resulting solution? (Assume that the volume change does not change upon adding hydrobromic acid)

Solution

up on adding 0.0538 / 0.25 = 0.2152 M HBr added

[NH4Br] = 0.217 + 0.2125 = 0.4322 M

[NH3] = 0.440 - 0.2125 = 0.2275 M

pKb of NH3 = 4.75

pOH = pKb + log [NH4Br] / [NH3]

pOH = 4.75 + log [0.4322] / [0.2275]

pOH = 5.03

pH = 14 - 5.03

pH = 8.97