In an experiment a buffer solution containing the acid HX and its conjugate base X-, has a pH=6.00. A 10.00 mL sample required 13.10 mL 0.1000 M NaOH to titrate the sample to the Basic equivalence point. 9.90 mL 0.1000 M HCl were required to titrate a second 10.00 mL buffer sample to the Acidic equivalence point. What is the pKa for the acid HX?
The pH of the buffer = 6.00
It can be calculated using the Hendersen equation as
pH = pKa + log [conjugate base] /[acid]
So we need to calculate the [conjugate base] and [acid]
1) basic equivalence point
HA + OH- -----------> A- + H2O
10mL x[acid] 13.10x0.10M
As this is the basic equivalence point
mmoles of acid = mmoles of base added
10mL [acid] = 13.10mL x 0.10M
hence [acid] in buffer = 0.131 M
2) acidic equivalence
A- + H+ ---------> HA
10.0 x[conj.base] 9.9mLx0.1
at acid equivalence
mmoles of conjugate base = mmoles of acid added
10.0mL x [conjugate base] = 9.9mL x0.1M
thus [conjugate base] = 0.099 M
3) now pKa
we have pH = pKa + log [conjugate base]/[acid]
6.00 = pka + log(0.099)/(0.131)
thus pKa = 6.1216
The pKa of the acid HA = 6.1216
Get Answers For Free
Most questions answered within 1 hours.