Question

Buffer capacity is a measure of a buffer solution\'s resistance to changes in pH as strong acid or base is added. Suppose that you have 175 mL of a buffer that is 0.440 M in both hydrofluoric acid (HF) and its conjugate base (F–). Calculate the maximum volume of 0.460 M HCl that can be added to the buffer before its buffering capacity is lost.

THE ANSWER IS NOT 1.34 X 10^-4

Answer 1

pKa of HF = 3.17

perfect buffer : pH = pKa = 3.17

buffer capacity lost when pH change one unit

pH change = 3.17 - 1 = 2.17

initial millimoles of HF = F- = 175 x 0.440 = 77

F-    + HCl ------------------------------> HF + Cl-

HF = millimoles = 77 + 0.460 V

F- millimoles = 77 - 0.460 V

moles of F - / moles of HF = 0.1

77 - 0.460 V / 77 + 0.460 V = 0.1

77- 0.460 V = 7.7   + 0.046 V

   69.3               = 0.506 V

V = 137 mL

volume of HCl needed = 137 mL