[myelver10]

what is the pH of the solution that results from adding 15 mL of 0.50 M NaOH to 25 mL of 0.50 M HF?  Ka for HF = 7.2 * 10^-4      

[AWK]

Hint - this is a buffer solution

Warning - HF is a weak acid

[pizza1512]

Hint - this is a buffer solution

What that?.



 

[jdurg]

It's a buffer solution because there's not enough NaOH to fully neutralize the HF in the mixture.  As a result, you wind up with a mixture of HF and NaF which is a weak acid and a salt of its conjugate base.  That equals a buffer.

[Kong]

very good.
A common misconception is that chemistry students think that once an acid/base is neutralized....it is neutral.   Wrong.  Only strong acids and bases are actually neutral.  Meaning the conjugates of strong acid/bases are neutral.   HF is a weak acid so its conjugate F- is a weak base.  
AS

[jdurg]

Very true as well.  Though in this case, if there was enough NaOH to fully react with the HF I'd feel safe in calling the overall mixture 'neutral'.  Mostly because I don't think the F- ion will tend to pull an H+ off of water and reform HF all too frequently.  If it did, then nearly all fluoride solutions would attack glass.