[hiro]

to determine the pka of chloroacetic acid, a quantity of 0.1M sodium hydroxide is added to 50ml 0.1Mchloroacetic acid, and the pHmeasured. from the measured pH, 2.85, it is known that pka for chloroacetic acid must be 2.85. wt volume (in ml) of 0.1M sodium hydroxide was added to the 50ml 0.1Mchloroacetic acid?

i can't find the volume...

[AWK]

A buffer solution that shows pH=pKa contains the same number of moles, both a salt and an acid.

[hiro]

mm...i am not good at cal..
do u mean the ans is actually 50ml? same as the quantity of chloroacetic acid?

[Borek]

Yes, that's what he meant. (Later edit: sorry, that's not what he meant, my mistake! It should be half the volume, read my post #7 below.)

Search the net for Henderson-Hasselbalch equation.

[GCT]

to determine the pka of chloroacetic acid, a quantity of 0.1M sodium hydroxide is added to 50ml 0.1Mchloroacetic acid, and the pHmeasured. from the measured pH, 2.85, it is known that pka for chloroacetic acid must be 2.85. wt volume (in ml) of 0.1M sodium hydroxide was added to the 50ml 0.1Mchloroacetic acid?

i can't find the volume...

when pH=pKa you're at the half equivalence point,

It should now be straightforward, relate half equilvanece point to equivalence point where all of the acid has been neutralized by a given volume of the strong base.

[hiro]

when pH=pKa you're at the half equivalence point,

It should now be straightforward, relate half equilvanece point to equivalence point where all of the acid has been neutralized by a given volume of the strong base.

>.<sorry....my eng is not good...
do you mean half equivalence point = weak acid neutralized by a given volume of the strong base? <--must be strong base/ acid?

[GCT]

>.<sorry....my eng is not good...
do you mean half equivalence point = weak acid neutralized by a given volume of the strong base? <--must be strong base/ acid?

no...it would be half right???  At equilvalence point, all of the strong base has reacted with the acid.

Thus at half equivalence point, the volume of the base corresponds to where half of the acid has reacted.  

Thus

-you'll need to find the moles of the acid and divide by 2.

-this will be the moles of Na0H, you can easily find the volume by multiplying by the molarity.  Got it?

[Borek]

OK, here goes exact explanation, before you will get confused more...

For a buffer solution containing acid and conjugated base (in this case acid is chloroacetic acid and conjugated base is CH₂ClCOO^- ion) pH is described by  Henderson-Hasselbalch equation:

pH = pKa + log([base]/[acid])

If concentration of acid is the same as concentration of conjugated base ratio of their concentrations equals 1 and logarithm of 1 is 0. Thus the equation takes form

pH = pKa

Now, in the question it is told that pH = pKa, so the ratio of concentrations must be 1. To obtain this ratio you must add enough NaOH to neutralize exactly half of the acid. As both solutions have the same concentration volume of the added NaOH solution must equal half of the volume of the original solution.

Sorry for my earlier misleading post