Concentration of both buffers is the same, only the pH is different

[Sayantan21]

Homework Statement

You are given two buffers [a) 0.1 M phosphate buffer of pH 7.7 and (b) 0.1 M phosphatebuffer at pH 6.71 . If acid is to be added to the buffers, which of them, do you think, will resist the pH changes better

Relevant Equations

Henderson Hesselbach Equation
pH = pKa + log([ conjugate base] /[ acid])

Here I used Henderson Hesselbach equation in both the cases , I.e
pH= pKa + log( [(PO4)3-]/ [H3PO4]) here pKa of phosphoric acid is 2.16 but the problem Is in both the cases the log part is same and Pka is also same so both will cancel out, and how can we solve? It is confusing.

 

[mjc123]

Phosphate is complicated because phosphoric acid is tribasic, so there are several equilibria. (See e.g. Wikipedia article on phosphoric acid.) In the region of pH 7, you only really need to consider the equilibrium between H₂PO₄^- and HPO₄^2-. 0.1M is the total concentration of phosphate species; the individual species will change with pH.

 

[Sayantan21]

So how will I know which will resist the pH better? Both are same buffers, with same Pka value

 

[Borek]

http://www.chembuddy.com/?left=buffers&right=pH-change

 

[epenguin]

You do know buffering power is maximum at pH = pK. ?
So that might be an easy question, which buffer is at the pH nearer to the pK?
However if I am not mistaken they are both equally distant.
So it is a question of in which direction are you going - and is adding acid taking you into a region of greater buffering or less?