What Is the pH of a Buffer System with H2PO4- and HPO42-?

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Discussion Overview

The discussion revolves around calculating the pH of a buffer system created by mixing H2PO4- and HPO42- in water. Participants explore the implications of the acid dissociation constants (Ka) and the behavior of the species in the buffer system.

Discussion Character

  • Technical explanation
  • Mathematical reasoning
  • Debate/contested

Main Points Raised

  • One participant presents a calculation using the Henderson equation to estimate the pH at 7.21, based on the assumption that H2PO4- will not act as a base due to Kb1 and that HPO42- will not act as an acid due to Ka3.
  • Another participant challenges the initial reasoning, asserting that H2PO4- will act as a base and HPO42- will act as both an acid and a base, questioning the extent of their roles in the buffer system.
  • A later reply acknowledges the initial participant's work as fine but does not address the disagreement about the roles of H2PO4- and HPO42-.

Areas of Agreement / Disagreement

Participants express differing views on the roles of H2PO4- and HPO42- in the buffer system, indicating that the discussion remains unresolved regarding their behavior as acids or bases.

Contextual Notes

The discussion includes assumptions about the behavior of the buffer components based on their Ka/b values, but these assumptions are not universally accepted among participants.

yolo123
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Hello Forum!

1g of H2PO4- and 1 g of HPO42- are put together into 100 ml of H2O. What is the pH of the buffer created.

Ka1= 7.5x10^-3
Ka2=6.2x10^-8
Ka3=4.8x10^-13


______________________
Okay. I uploaded my solutions. (Please disregard the part 1.184 g/mol and the 34% on my answers sheet. That was part of another problem.) Basically, I decided to put down all the reactions and say that H2PO4- and HPO4- will be the important reactions according to Ka/b values.
Then, I used Henderson to get 7.21.
Are my steps of thinking that H2PO4- will not act as base because of Kb1, and that HPO4- will not act as acid because of Ka3 necessary and correct? Are they part of the reasoning that I should put on, say an exam?

Many thanks.
 

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Your handwriting is hard to read. And of course H2PO4- will be acting as a base in the system; the question is just to what extent. Same with HPO42-. This is both an acid and a base.
 
Yes, I looked at it from the point of view of the values of the Ka/b values. Does it seem to make sense? Sorry about my handwriting. This is actually my third "clean" version I made just for the forum.
 
Your work looks fine.
 

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