PH of buffered solutions?

Question

Calculate the pH after 0.010 mol gaseous HCl is added to 250.0m of each of the following buffered solution:

0.050M NH3/0.15M NH4CL

Answer 1

Calculate moles of each component.

moles NH4+ = M NH4+ x L NH4+ = (0.15)(0.2500) = 0.0375 moles NH4+

moles NH3 = M NH3 x L NH3 = (0.050)(0.2500) = 0.0125 moles NH3

If HCl is added to this buffer, it will react with the buffer's alkaline component, NH3.

moles . . . . . .NH3  +  H+  ==>  NH4+

initial .. . . . .0.0125. . 0.010. . . 0.0375

change. . . .-0.010. . -0.010 . . +0.010

final. . . . . . .0.0025 . . .0 . . . . .0.0475

The Henderson-Hasselbalch equation is used for buffer problems.

pH = pKa + log ([NH3] / [NH4+] or equivalently

pH = pKa + log (moles NH3 / moles NH4+)

since both components are in the same solution.

pKa NH4+ = Kw / Kb NH3 = 1 x 10^-14 / 1.8 x 10^-5 = 5.6 x 10^-10

pH = -log (5.6 x 10^-10) + log (0.0025 / 0.0475) = 9.25 - 1.28 = 7.97