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Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: cloveryeah on March 18, 2015, 09:58:35 AM

Title: calculate pH of buffer solution
Post by: cloveryeah on March 18, 2015, 09:58:35 AM

(i) 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?? 

Title: Re: calculate pH of buffer solution
Post by: magician4 on March 20, 2015, 12:53:03 AM

given that die Ka values 1, 2 are by the factor of approx. 10 to the power of 3 apart, the meaning is that "the stronger acid" ( or the first dissociation step, to be more precise) wont be influenced in it's behaviour by the second stage at all, for all practical purposes

hence, for the problem at hand , we can treat the phthalic acid as a monoprotonic acid, and ignore the second acid function completely.

 :rarrow: you're right about "the pH ( in question) depending just on Ka1"

regards

Ingo