Question

1) A buffer solution contains 0.388 M NH3. Determine the pH change when 0.088 mol NaOH is added to 1.00 L of the buffer.

2) Determine the pH change when 0.070 mol HBR is added to 1.00 L of a buffer that is 0.49u M HF and 0.302 M in F-.

Please show work

Answer 1

1) Buffer of weak base/conjugate acid : NH3/NH4+

Lets say we had equal concentrations of [NH3] = [NH4+] = 0.0388 M in the buffer initiallly,

initial pH of buffer = pKa of NH4+ = 9.25

When 0.088 mol NaOH has been added to 1 L buffer,

new [NH3] = (0.388 M x 1 L + 0.088 mol)/1 L = 0.476 M

new [NH4+] = (0.388 M x 1 L - 0.088 mol)/1 L = 0.3 M

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

pH = 9.25 + log(0.476/0.3) = 9.45

change in pH = final - initial = 9.45 - 9.25 = 0.2

2) Initial pH of buffer HF/F- (weak acid/conjugate base)

pH = pKa + log(F-/HF)

     = 3.17 + log(0.302/0.490)

     = 2.96

When 0.070 mol HBr is added to 1 L of buffer

new [F-] = (0.302 M x 1 L - 0.070 mol)/1 L = 0.232 M

new [HF] = (0.490 M x 1 L + 0.070 mol)/1 L = 0.560 M

new pH = pKa + log(F-/HF)

pH = 3.17 + log(0.2326/0.560) = 2.79

change in pH = final - initial = 2.79 - 2.96 = -0.17