Question

A buffer solution contains 0.388 M ammonium bromide and 0.385 M ammonia.

If 0.0484 moles of perchloric acid are added to 225 mL of this buffer, what is the pH of the resulting solution?
(Assume that the volume does not change upon adding perchloric acid.)

pH =

Please illustrate with step-by-step answer,

thank you.

Answer 1

after adding perchloric acid following reaction takes place

HClO4 + NH3 ---------> NH4⁺

so NH3 will decrease and NH4⁺  will increase.

molarity of perchloric acid added = 0.0484 / 0.225 = 0.215 M

now concentration of NH3 = 0.385 - 0.215 = 0.17 M

NH4⁺   = 0.388 + 0.215 = 0.603 M

mixture of NH3 and NH4⁺  act as basic buffer

for this buffer

pOH = pKb + log [NH4⁺] / [NH3]

pKb of NH3 = 4.75 standard value

pOH = 4.75 + log [0.603] / [0.17]

pOH = 4.75 + 0.55

pOH = 5.3

pH = 14 - 5.3

pH = 8.7