How to Determine the pH of a Buffer Made with K2HPO4 and KH2PO4?

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Discussion Overview

The discussion revolves around calculating the pH of a buffer solution composed of K2HPO4 and KH2PO4. Participants explore the dissociation of these compounds, their roles as acid and base, and the application of the Henderson-Hasselbalch equation in this context.

Discussion Character

  • Homework-related
  • Mathematical reasoning
  • Conceptual clarification

Main Points Raised

  • One participant questions how K2HPO4 and KH2PO4 dissociate in solution and which acts as the acid or base, expressing uncertainty about their conjugate pairs.
  • Another participant suggests writing stepwise dissociation reactions for the multiprotic acid to clarify the situation.
  • A third participant states that KH2PO4 is considered the acid and K2HPO4 the conjugate base, providing a calculated pH of 7.4 using a given Ka value.
  • A later reply confirms the pH calculation but prompts further understanding of why H2PO4- is classified as an acid.

Areas of Agreement / Disagreement

Participants generally agree on the classification of KH2PO4 as the acid and K2HPO4 as the base, but there remains some uncertainty regarding the dissociation process and the reasoning behind these classifications.

Contextual Notes

The discussion includes assumptions about the dissociation of multiprotic acids and the application of the Henderson-Hasselbalch equation, but does not resolve the details of the dissociation steps or the implications of the chosen Ka value.

Who May Find This Useful

Students and educators in chemistry, particularly those studying buffer solutions and acid-base equilibria.

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Homework Statement



Calculate the pH of a buffer made with 10.0 g of K2HPO4 and 5.0 g of KH2PO4 in 1.0L of water.

Homework Equations



-log Ka + log (base/acid) = pH

The Attempt at a Solution



So...mathwise, I know how to do this problem. My only question is, aren't K2HPO4 and KH2PO4 both salts? How would they break up? I don't know which would be the acid or the base, and I couldn't find a Ka/Kb value to help me.

Basically I just can't figure out how they break up and what their conjugates are... :p
 
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Ignore cation. Write stepwise dissociation reactions for multiprotic acid.
 


So my teacher said that the KH2PO4 would be considered the acid, and that the K2HPO4 would form a strong conjugate base. I converted from grams to mols, and then to molarity. Using the given Ka value of 6.2 x 10^-8, I just plugged everything into the Henderson-Hasselbalch equation and got a pH of 7.4. Does this sound right? :/
 


Yes.

Do you understand why H2PO4- is an acid?
 

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