H3PO4 Titration Problem: Determining Molar Concentration and Indicator Choice

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Discussion Overview

The discussion revolves around a titration problem involving phosphoric acid (H3PO4) and sodium hydroxide (NaOH). Participants are attempting to determine the molar concentration of the acid based on the volume of titrant used and are debating the appropriateness of bromophenol blue as an indicator. The conversation includes aspects of acid-base theory, titration curves, and the definition of equivalence points.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant presents a titration scenario and asks for confirmation of their calculation regarding the molar concentration of H3PO4 and the suitability of bromophenol blue as an indicator.
  • Another participant questions the correctness of the equivalence point calculation and discusses the nature of the titration curve, suggesting there may be multiple equivalence points.
  • A participant asserts that the definition of the equivalence point involves equating moles of titrant and analyte, leading to the conclusion that all H3PO4 converts to H2PO4-.
  • There is a claim that H2PO4- can act both as an acid and a base, and a challenge to the calculated pH at the first equivalence point.
  • One participant expresses confusion regarding the formulas referenced in an article and questions how to arrive at a specific pH value.
  • A link to an external resource is shared, which presents a different approach to calculating pH, raising questions about its accuracy.
  • Another participant asserts that the theoretical pH at the first equivalence point is 4.70, contradicting earlier calculations.

Areas of Agreement / Disagreement

Participants express differing views on the calculation of the equivalence point and the resulting pH values. There is no consensus on the accuracy of the calculations or the appropriateness of the chosen indicator.

Contextual Notes

Participants reference various assumptions about the behavior of phosphoric acid and its conjugate base, as well as the definitions of equivalence points, which may not be universally agreed upon. The discussion also highlights potential gaps in participants' understanding of the relevant formulas and concepts.

yolo123
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Hello Forum,
Another problem again... Please bare with me.

You are titrating an H3PO4 solution of acid. You titrate 20ml of that solution. You have used 15 ml of 0.500 M NaOH at the equivalence point. What is the molar concentration of the acid? Would bromophenol blue (pH 3.0-4.6) be appropriate to use as an indicator? (ie, find the pH)

Please look at my steps. Would you get the same answer as me?

My acid base theory is really wobbly and I need to perfect it. I want to make sure I have a strong foundation, especially because I want to take organic chemistry (but I need to get at least 75% in my current class :S)
 

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I am not convinced your calculation of equivalence point is correct. You have a solution of an amphiprotic salt NaH2PO4.

This is about shape of titration curve - is there only one, or two separate equivalence points? Once you know equivalence point pH you should check how much of the H2PO4- is neutralized at this pH. If almost none, that's OK, if too much - there will be no well visible, separate inflection points.

Actually there are two, see the titration curve.

attachment.php?attachmentid=69775&stc=1&d=1400050786.png
 

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Is the definition of equivalence point not: moles of titrant=moles of analyte?
If so, then all H3PO4 becomes H2PO4- which will now act as an acid.
 
yolo123 said:
If so, then all H3PO4 becomes H2PO4- which will now act as an acid.

It also acts as a base. And pH at the first equivalence point is definitely not 3.94 that you calculated.
 
I read the article you linked. But, we never saw those kinds of formulas in class! What would I do to get that 4.70?
 
http://www.occc.edu/clvahlberg/documents/chem1215/Unit%204%20-%20Equilibria/phosphorc%20acid%20eq%20points.pdf

These guys did it similarly to me. Are they wrong?
 
Last edited by a moderator:
yolo123 said:
I read the article you linked. But, we never saw those kinds of formulas in class! What would I do to get that 4.70?

Not sure what the question is - just use formulas derived there. There is even a table that shows how the accurately calculated pH differs form the approximated result calculate with the derived formula exactly for your solution.

yolo123 said:
http://www.occc.edu/clvahlberg/documents/chem1215/Unit%204%20-%20Equilibria/phosphorc%20acid%20eq%20points.pdf

These guys did it similarly to me. Are they wrong?

Unfortunately yes, they are wrong. Theoretical pH at the first equivalence is 4.70.
 
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