What is the pH of a Soft Drink with NaH2PO4 and Na2HPO4?

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Discussion Overview

The discussion revolves around calculating the pH of a soft drink that contains sodium dihydrogen phosphate (NaH2PO4) and disodium hydrogen phosphate (Na2HPO4) as buffer components. Participants explore the approach to solving the problem involving weak acids and weak bases in a buffer solution.

Discussion Character

  • Homework-related
  • Conceptual clarification
  • Technical explanation

Main Points Raised

  • One participant expresses difficulty in understanding how to approach the problem of a mixture of weak acid and weak base, specifically seeking guidance on the procedure.
  • Another participant suggests that the key concept is to focus on the significant equilibrium between H2PO4(-) and HPO4(2-), while largely ignoring the other deprotonations of phosphoric acid.
  • The same participant recommends using the Henderson-Hasselbalch equation for the calculation, emphasizing the importance of selecting the correct acid dissociation constant (Ka).
  • There is a mention that including additional equilibria may complicate the solution without significantly affecting the outcome.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the best approach to solve the problem, as one participant is seeking clarification while another provides a potential method. The discussion remains unresolved regarding the overall procedure to follow.

Contextual Notes

There is uncertainty regarding the treatment of multiple equilibria in the buffer system, and participants have not fully explored the implications of including or excluding certain equilibria in their calculations.

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Hi

I have been trying several days to solve this problem. But i can't find the procedure to solve this weak acid and weak base problem. I will really appreciate some help.

What is the pH of a soft drink in which the major buffer ingredients are 6.90 g of NaH2PO4 and 5.70 g of Na2HPO4 per 355 mL of solution?

Thanks
 
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Hi

The problem relies in the fact i don't know how to solve for weak acid with a weak base. I do know how to buffers with weak acid and salt or weak base and salt. But I have no idea where to start when you are asked a mixture of weak acid and weak base at the same time. I will really appreciate your help. I don't need the final answer just a way to solve this kind of problem.

Thanks
 
I think that the conceptual key here is that you can largely ignore the first and third deprotonations of phosphoric acid. If H2PO4(-) and HPO4(2-) exist in significant concentration, then the concentrations of H3PO4 and PO4(3-) will be negligible. That is to say, the only significant equilibrium here is

H2PO4(-) <-> H(+) + HPO4(2-), all phases aqueous.

Then you can just apply the Henderson-Hasselbach equation. Just make sure you use the right Ka.

If you feel paranoid and want to include all of the other equilibria, you can. I'm guessing it'll just be extra work for a 10^-18 or so correction.
 

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