To determine if a buffer is formed, you need to identify a weak acid and its conjugate base (or a weak base and its conjugate acid) among the choices. In this case, the only weak acid listed is HF. Therefore, the only option that has the potential to form a buffer is option e) 0.040 mol HF.
To understand why HF can act as a weak acid and form a buffer, you need to know its dissociation reaction in water:
HF + H2O ↔ H3O+ + F-
The presence of both HF and F- in the solution allows for the formation of a buffer system. HF can donate H+ ions, while F- can accept H+ ions. This ability to resist changes in pH is what characterizes a buffer.
On the other hand, HCl and NaOH are both strong acids and bases, respectively, meaning they are completely ionized in water and do not form buffers. The other options (a). 100 mol HCl, b). 0.060 mol HCl, and c). 0.040 mol HCl) are also strong acids and would not form buffers either.
In summary, the only choice that has the potential to form a buffer when added to 250.0 mL of 0.150 M SnF2 is option e) 0.040 mol HF.