Which of the following would for a buffer if added to 250.0 mL of .150 M SnF2?
a).100 mol HCl
b).060 mol HCl
c).040 mol HCl
d).040 mol NaOH
e).040 mol HF
I figured I could find out the pH of the solution with HCl/NaOH/HF added, and then find the pH when equal amounts of H+ and OH- are added (respectively), and if only a small pH change occurs I'd know if I had a buffer. Does this seem like more work than necessary to do the problem?
On another note, I'm having trouble setting things up. I believe SnF2 is basic, so I would assume that in water it would create a reaction like this:
H2O + SnF2 --> OH- + HSnF2+
But I really don't know if that's right or not. I wanted to find the pH of HCl as well, but I can't find a Ka value for it in my book (I was going to set it up as Ka= x^2/1.00 m) is this the wrong way to approach it?
In answer to your first question, YES, it is more work than necessary. The simple way.
What is a buffer? It is a weak acid and a salt of the weak acid OR a weak base and a salt of the weak base. So you have SnF2 in the problem. That is a salt. All you need to do is to look at the choices Anything other than HF isn't right.Besides, HF is the ONLY weak acid in the list of choices.
HCl doesn't have a Ka listed because it is a strong acid (100% ionized). NaOH, likewise, is a strong base and it is completely ionized.
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.