Acid base question

[bajoneswadup]

Actually 2 questions:

1) the half equivalence point of a strong by strong titration, is the ph the pka of the acid? TBR said that works only for strong by weak titrations, but EK 1001 says it is... CONFUSED!

2) Why is HCl/ KCl not a good buffer? Again, TBR says buffers = weak&weak or strong&weak. EK says a buffer is only weak/weak. Now obviously HCl/KCl wouldn't be ideal b/c hcl is so strong and kcl sucks, but sometimes if you want to lower the ph of something you would use it... anyways, confused.

Somebody help me understand this without completely making me feel like I have to relearn everything. Last thing I need now is to feel like I don't understand easy titration questions a month before the mcat lol. Thanks

---

Members don't see this ad.

 

[411309]

Actually 2 questions:

1) the half equivalence point of a strong by strong titration, is the ph the pka of the acid? TBR said that works only for strong by weak titrations, but EK 1001 says it is... CONFUSED!

2) Why is HCl/ KCl not a good buffer? Again, TBR says buffers = weak&weak or strong&weak. EK says a buffer is only weak/weak. Now obviously HCl/KCl wouldn't be ideal b/c hcl is so strong and kcl sucks, but sometimes if you want to lower the ph of something you would use it... anyways, confused.

Somebody help me understand this without completely making me feel like I have to relearn everything. Last thing I need now is to feel like I don't understand easy titration questions a month before the mcat lol. Thanks

1. In both titrations the half equiv point is where ph=pka

2. KCl is a salt and HCl is a strong acid I can't remember why this wont make a good buffer, anyone care to chip in?

 

[milski]

1. In both titrations the half equiv point is where ph=pka

2. KCl is a salt and HCl is a strong acid I can't remember why this wont make a good buffer, anyone care to chip in?

You need some not disassociated HCl for the buffer to work. pKa of HCl is so low that it makes it impossible/impractical.

 

[411309]

i just referred back to TBR. couldnt you use HCl and a weak base to buffer though?

 

[bajoneswadup]

Yeah but is there any other buffer exceptions I should know about? HBr same? and HI? I mean they're stronger than HCl... Is only nitric and sulfuric
acid only strong acuds that will make a good buffer?

What about bases too? OMG ok now I'm confused :/

 

[411309]

I think it's because you cant buffer using a strong acid and conjugate salt, it would have to be a weak acid and conjugate salt. If you buffered with a strong acid it would be 1/2 equiv strong acid with weak base.

correct me if im wrong though, someone. i hated this chapter, lol.

 

[milski]

You need the acid and its conjugate to be in the solution for the buffer to be effective. Ideally, in the same concentration, although up to 1:10 is still considered a buffer. That cannot work with a strong acid.

 

[bajoneswadup]

You need the acid and its conjugate to be in the solution for the buffer to be effective. Ideally, in the same concentration, although up to 1:10 is still considered a buffer. That cannot work with a strong acid.

So for the mcat we assume that all strong acids, including h2so4 and hno3, and the conjugate can't be buffers. And all strong bases with it's conjugate can't either. Right?

 

[milski]

So for the mcat we assume that all strong acids, including h2so4 and hno3, and the conjugate can't be buffers. And all strong bases with it's conjugate can't either. Right?

Not only for MCAT, you cannot make a practical buffer with a strong acid.

 

[MedPR]

According to TBR you can make a buffer with equal molar equivalents of weak base + conjugate acid as well as weak base (or acid) with 1/2 equivalent of strong acid (or base).

 

[milski]

According to TBR you can make a buffer with equal molar equivalents of weak base + conjugate acid as well as weak base (or acid) with 1/2 equivalent of strong acid (or base).

Hm.. for the weak acid case: the strong base will take some of the [H] in [HA]<->[H]+[A] and you'll end up with some extra conjugate of the weak base, effectively making it a buffer.

I was thinking only of having the strong acid and its conjugate but yes, that way works too, it's another way to get both weak acid and its conjugate together in a solution.

(acid/base can be replaced in most of what I said)

 

[bajoneswadup]

I don't get why strong bases would be any better of a buffer than using a strong acid, both using w/ conjugates.. I mean, I guess it makes sense of what you were saying, it's likely to be over 10:1 ratio if they don't specify an amount, but honestly I think that question was silly the more I think about it..

 

[Class1P]

Just because you base is weak doesn't mean your strong acid still won't dissociate completely.

 

[syoung]

Let me get this straight in my mind
HA is a strong acid
ha is a weak acid and a- is the conj base
If I had only a- I can add HA to it (HA will dissociate completely) and I will get a- and ha (in whatever stoichiometric amounts). Now I have a buffer (weak acid and conj base) correct?
Vice versa for:
B- is a strong base
b- is a weak base and hb is the conj acid

Of course we also have just ha and conj salt (like Na/a) this would also make a buffer depending on the stoichiometric amounts

Do I have this right?