H3PO4 Titration Problem: Determining Molar Concentration and Indicator Choice

[yolo123]

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)

 

[Borek]

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.

 

[yolo123]

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.

 

[Borek]

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.

 

[yolo123]

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?

 

[yolo123]

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?

 

[Borek]

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.

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.