What is the pH of a 0.1M Na2HAsO4 solution?

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Discussion Overview

The discussion revolves around calculating the pH of a 0.1M Na2HAsO4 solution, which involves understanding the acid dissociation constants of the polyprotic acid H3AsO4. Participants explore different approaches and formulas related to buffer solutions and pH calculations.

Discussion Character

  • Homework-related
  • Mathematical reasoning
  • Debate/contested

Main Points Raised

  • One participant presents the problem statement, including the relevant acid dissociation constants for H3AsO4.
  • Another participant mentions a formula for pH involving the average of pKa1 and pKa2, suggesting it as a possible approach.
  • A third participant challenges the applicability of the buffer formula to the current problem, indicating a disagreement about its relevance.
  • There is a correction offered regarding the previously mentioned formula, suggesting it is close but not accurate.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the correct approach to calculating the pH, with multiple competing views on the relevance of the buffer formula and its application to the problem.

Contextual Notes

Participants express uncertainty about the correct method to apply, and there are unresolved aspects regarding the use of the pKa values and their relationship to the pH calculation.

Nikitin
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Homework Statement



The cid constants for the polyprotic acid H3AsO4 are Ka1=5x10^-3, Ka2=8x10^-
8, Ka3=6x10^-10. What is the pH of a 0.1M Na2HAsO4 solution?

Homework Equations


Kb = [H2A]*[OH]/[HA]
mass balance, charge balance.. etc.


The Attempt at a Solution



Well I've tried setting up a bunch of equations but I can't do this. Does anyone have any tips?
 
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never mind.. I asked a classm8 and he told me about the pH=pka1+pka2 /2 equation.
 
For future, you may want to remember that formula as the pH of buffers.
 
AGNuke said:
For future, you may want to remember that formula as the pH of buffers.

No, these things are completely unrelated.

Nikitin said:
never mind.. I asked a classm8 and he told me about the pH=pka1+pka2 /2 equation.

Close, but wrong.
 

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