Estimating pH for Highest Buffer Capacity from HO2C-C6H4-CO2H & HO2C-C6H4-CO2Na

[cloveryeah]

Ka values of the 1st and 2nd dissociation of phthalic acid (HO2C-C6H4-CO2H) are
given below. Estimate the pH value for highest buffer capacity when a buffer is
prepared from 1:1 mixture of phthalic acid and sodium hydrogen phthalate (HO2CC6H4-
CO2Na) in water?

HO2C-C6H4-CO2H <=> H+ +HO2C-C6H4-CO2- Ka1=1.3X10^-3
HO2C-C6H4-CO2- <=> H+ + -O2C-C6H4-CO2- Ka2=3.1X10^-6

so i m wondering that whether the pH (we want to find when buffer capacity is the highest) is just depends on Ka1...?

or Ka2 is counted also??

 

[Borek]

What definition of buffer capacity are you expected to use?

 

[epenguin]

Here you have an equimolar mixture of an acid and its monobasic salt. What pH does that give normally?

Then you have the complication that the acid has a second dissociation.
At the pH you calculated, how much lf the ^-CO₂-C₆H₄-CO₂^- have you got?
Note you are not asked to calculate exactly, only to "estimate". Understanding what approximations you can use and when is an important part of this general subject.