[Custard]

Hi all, this one problem on the topic in the subject has me stumped:

You have the following reagents on hand:

Solids (pKa of Acid Form is Given)	Solutions (pKa Given if applicable)
Benzoic acid (4.19)	5.0M HCl
Sodium acetate (4.74)	1.0M acetic acid (4.74)
Potassium fluoride (3.14)	2.6M NaOH
Ammonium chloride (9.26)	1.0M HOCl (7.46)

What combinations of reagents would you use to prepare buffers at the following pH values?
a) 3.0   b) 4.0   c) 5.0   d) 7.0   e) 9.0

I'm pretty sure that I need to use Henderson-Hasselbalch here and match up the pKa's with pH's but kinda fuzzy on the details.  Thanks in advance.

[Borek]

If you add some NaOH to benzoic acid solution, what will happen?

If you add some strong acid to sodium acetate, what will happen?

[Custard]

Right, I get that adding a strong base like NaOH will result in a reaction looking something like this:
HA + OH --> A+H2O
Obviously that will increase the pH since the [H] has decreased, and the inverse will pretty much  be what happens with something like HCl + sodium acetate.

I'm just not quite sure how to pair them up to get the desired pH.
For example, I know that to prepare a 3.0 pH buffered solution, I'll need to use potassium fluoride (since its pKa is close to 3.0) and an acid.  I'm just not sure how to figure out which acid.

[UG]

You will want to use a strong acid so that all of the F^- ions are converted into HF as required

[Custard]

Ah, I see now.  Thanks.