Question

Cacodylic acid (dimethylarsinic acid, HAsO2(CH3)2) is a common buffer used in biology to prepare and fix biological samples. The pKa of cacodylic acid is 6.3.

Part A What is the pH of a buffer solution prepared by mixing 20.0 mL of 0.0700 molL−1NaOH with 20.0 mL of 0.170 molL−1 cacodylic acid? Express your answer using one decimal place. pH =

Part B What is the pH of the buffer solution in part A after 0.0002 moles of H3O+ was added to the mixture? Express your answer using one decimal place. pH =

Answer 1

Sol.

Part A :

initial moles of HAsO2(CH3)2 = Concentration of HAsO2(CH3)2 × Volume of HAsO2(CH3)2 / 1000

= 0.170   × 20 / 1000  

= 0.0034 mol

initial moles of NaOH ( OH-)

= concentration of NaOH × Volume of NaOH / 1000

= 0.0700  × 20 / 1000  

= 0.0014 mol  

Reaction :

HAsO2(CH3)2 + OH- ----> AsO2(CH3)2- + H2O

initial 0.0034 0.0014 0

change - 0.0014 - 0.0014 + 0.0014

equilibrium 0.0020 0 0.0014

As pKa = 6.3  

So , Using Henderson - Hasselbalch equation,  

pH = pKa + log ( moles of AsO2(CH3)2- / moles of HAsO2(CH3)2 )

= 6.3 + log ( 0.0014 / 0.0020 )

=   6.1  

Part B :

As 0.0002 moles of H3O+ added due to which moles of HAsO2(CH3)2 increases and moles of AsO2(CH3)2- decreases . So ,

Reaction :

    AsO2(CH3)2- + H3O+ ----> HAsO2(CH3)2 + H2O

initial   0.0014 0.0002 0.0020

change - 0.0002 - 0.0002 + 0.0002

equilibrium 0.0012 0 0.0022

Therefore , again , Using Henderson - Hasselbalch equation ,

pH = 6.3 + log ( 0.0012 / 0.0022 )

=   6.0