[Nidea]

Hello everyone  

I have a problem I am trying to work out:

78mL of 0.100 M acetic acid solution and 20mL of 0.50 M sodium hydroxide are run from burettes into a 150mL volumetric flask, and the soultion made up to the mark. What is the pH of the solution? (given Ka for acetic acid is 1.75x10-5)

I don't have much of an idea how to work this kind of question out. Can anybody get me started? Any help would be very much appreciated.

Thanks, Nidea.

[AWK]

Write down a neutralization reaction.
calculate an excess of reagent (NaOH)
Calculate concentration of this amount of NaOH in 150 mL
Finally calculate pH of NaOH solution

In the presence of NaOH excess you can neglect sodium acetate from calculations
By the way, I've never seen a volumetric flask of such volume!

[ksr985]

here goes nothing

pH=12.16

[Dude]

12.16 is correct.  After the equivalence point, the pH for a weak acid / strong base is calculated similar to a strong acid / strong base.

See "Analytical Chemistry: An Introduction", 5th ed, Skoog, DA; West, DM; Holler, FJ, Saunders, 1990, Ch 11, p 192 for an almost identical sample problem.