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suppose you performed a titration of a weak acid and you found that the equivalence point occurred at 11.38 ml of added naoh. at what volume would you use the ph to determine the pka of the acid? enter your answer numerically to three significant figures.



Answer :

At volume 6.78ml, pH will be equal to pKa of the acid.

According to the Henderson- Hasselbach equation,

pH= pKa + log {A-}/ {HA}

where {A-} is the concentration of conjugate base and

{HA} is the concentration of weak acid

At the equivalence point, the volume of base added is just enough to exactly neutralize all the acid.

At one half of this volume of added base, called the half equivalence point, enough has been added to neutralize half of the acid. Since, half of the acid reacted to form A-, the concentration of A- and HA-, at the half equivalence point are the same.

ph= PKA+log1

pH= pKa (log1=0)

Therefore, V eq = 13.57ml

V 1/2 eq= 13.57/2

=6.78ml

Hence, at volume 6.78ml, pH will be equal to pKa of the acid.

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