M.Ed.,San Francisco State Univ.
Jonathan has been teaching since 2000 and currently teaches chemistry at a top-ranked high school in San Francisco.
Here are some tips and tricks for identifying if a solution is a Buffer solution. Now if you remember a Buffer solution is basically a solution that resists change in pH. And that's when I add an acid or base to that buffer and it resists change in pH.
How it does it, well you need to have one of two cases. It needs to have a weak acid and its conjugate base in the same solution. Or, I need to have a weak base, and it's conjugate acid in the same solution. So only those two. One thing that my students get tricked on, sometimes they don't realise that it can't be something with a strong acid or a strong base. It has to be a weak acid and its conjugate base or a weak base and its conjugate acid.
Let's take a look at some examples, just to help you out. If I have HF and NaF, Sodium Fluoride and they're the first solution that we have here. Is it a buffer? Well HF is a weak acid. So that first part and your number one, weak acid works. Now the question is, is it a conjugate base? Well the conjugate base of HF, well if we take away an H+ from HF in the formula, then the conjugate base is F-. Anyways, conjugate base is F-.
Now let's take a look at NaF. Now when we put that in water, because these are all aqueous, because in a Buffer a solution usually you'd have a water as your solvent. So I have NaF and using your solubility rules, you would know that these are soluble. What happens is, when you put then in water you actually ionize these. So you end up with Na+ ions and you end up with F- ions. Well hey F- and F-, those are the same. And so we actually do have, from the F- part, we do have the conjugate base of HF in there. And so yes, this one's a Buffer.
Let's try another one. Let's say I have HNO3 and I have NaNO3; Sodium Nitrate. We take a look here. HNO3 looks like an acid but it's a strong acid. Well right off the bat, you know that this can't be a Buffer because you have a strong acid already present in your solution. Remember, a Buffer is a weak acid and its conjugate base. So, not a buffer, because you have the strong acid there.
Let's take a look at a third case. Say if I have NH3 and I have NH4NO3; Ammonium Nitrate. So NH3 you figure that out? That's a weak base. I have one part. That means in the solution, I also need i's conjugate acid. Well how do I get the conjugate acid? Remember the shortcut for the conjugate acid is to add H+ to the formula of the weak base. NH3 plus and H, so NH3 the 3 will become a 4, and I'll put a plus here. So the conjugate acid is NH4+.
Well let's take a look here. Because of your solubility rules, you know that Ammonium Nitrate is soluble. And so what happens is, it makes ions, and so I end up with NH4+ and NO3-. Well hotdog you have NH4+ and then NH4+, that was the conjugate acid. And so yes, this is a Buffer.
Let's take a look at one more case, just to make sure, one more example. Say for example I have, say H2S and I have Na2S. Let's take a look here. Well H2S is a weak acid, okay so far so good, we have half of it. That means that we need its conjugate base. To get the conjugate base we take away an H from there, so H2 that becomes, so we have one H. Then we have HS and then we take away 1 because we take away +. So that will be HS-. The conjugate base is HS- for this.
Na2S that's also soluble in water and so it becomes 2Na+ plus one ion it's plus S2-. Well S2- and H2S not the same. Since S2- is not the conjugate base of H2S, then this is no, not a Buffer.
Hopefully these examples help you figure out some tips and tricks. Basically, the pattern is, if you have a weak acid and its conjugate base, or a weak base and its conjugate acid, then you have a Buffer. But make sure you don't get tripped up when you have a strong acid. Remember strong acids, strong bases, never ever make a Buffer. Hopefully this helps you out in identifying buffers. Have a good one.