# Homework Help: PH in buffer solution

1. Mar 18, 2015

### cloveryeah

• Please post in HW section and please use the HW template.
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??

2. Mar 18, 2015

### Staff: Mentor

What definition of buffer capacity are you expected to use?

3. Mar 19, 2015

### 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.

Last edited: Mar 20, 2015