Question

# In an experiment a buffer solution containing the acid HX and its conjugate base X-, has...

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

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