Creating a Buffer pH of 9.23: Step-by-Step Guide

[CivilSigma]

Hello, I have the following problem:

What volumes of 0.200 M HCOOH and 2.00 M NaOH would make 500.0 mL of a buffer with the pH of one made from 475 mL 0.200 M benzoic acid and 25 mL of 2.00 M NaOH?

I have determined that the pH of the buffer should be 9.23. Now, I am stuck on how I would assemble the buffer. Moreover, I determined that the ratio of the week conjugate base to week acid is 5.31 ( CH3COO / CH3COOH ).

What do I need to do next?

Thanks.

 

[Borek]

I have determined that the pH of the buffer should be 9.23.

I am afraid that's not what I got, What value of pKa for benzoic acid have you used? And how you got 9.23? Please show.

Moreover, I determined that the ratio of the week conjugate base to week acid is 5.31 ( CH3COO / CH3COOH ).

For the pH 9.23 that's orders of magnitude off. For the real pH of the buffer order of magnitude is OK, but the value is wrong. Besides, question asks for HCOOH, not CH₃COOH (and the ratio is wrong for both acids).