What Is the pH of a Strong Triprotic Acid Before Titration?

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The discussion revolves around determining the pH of a strong triprotic acid before titration with NaOH. Participants clarify that a strong acid dissociates completely in solution, leading to the calculation of moles of protons from the acid. The unknown acid's concentration is derived from the moles of acid and its volume, resulting in confusion over the pH calculation due to incorrect volume considerations. The conversation highlights the misconception that a negative pH is impossible, while also addressing the importance of accurate calculations in acid-base chemistry. Ultimately, the participants work through the complexities of the problem to arrive at a better understanding of pH in relation to strong acids.
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Homework Statement


24 ml of 1.5 M solution of unknown acid is titrated with 54 ml 2 M NaOH. The unknown acid is triprotic. If the unknown acid is strong, what is the pH of this acid before titration


Homework Equations



Mvn= Mvn to find that it was tri-protic.

The Attempt at a Solution


literally no idea where to begin
 
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What does it mean for an acid to be "strong"?
 
that it dissociates completely in solution
 
OK, how many moles of acid do you have?
 
.036 moles
 
And so how many moles of protons, when it dissociates?
 
i don't know... do you?
 
What does triprotic mean?
 
that the formula is H3___ (3 being a subscript) so its got 3 hydrogen
 
  • #10
OK, so if there are 0.036 moles of acid, how many hydrogens?
 
  • #11
.012 or .108??
 
  • #12
If a cake needs 3 eggs, and I have 4 cakes, how many eggs do I need?
 
  • #13
12? so .012?
 
  • #14
How did you get 12 to my question?

Consider that you have 0.036 cakes, each containing 3 eggs...
 
  • #15
so .108 i did .012 because i thought you were saying 12 egga for 4 cakes
 
  • #16
ah, sorry, probably a bad choice of numbers on my part then :)

OK, 0.108 moles of protons. Can you define pH in terms of concentration?
 
  • #17
ya - [log concentration]
 
  • #18
And what is the concentration? (given that you know number of moles and volume)
 
  • #19
1.38, but that's too high the negative log of that is negative itself
 
  • #20
How have you got 1.38? Also, negative pH is not a problem either.
 
Last edited:
  • #21
borek said moles over volume. Therefore with .108 moles over the volume of .024L i was wrong with 1.38 because i added the volume of the NaOH as well but this number is still to high too.
 
  • #22
Why do you think it's too high?
 
  • #23
cause you get a negative pH
 
  • #24
And that's a problem because?
 
  • #25
there is no such thing as a negative pH...
 
  • #27
i have the sme question! nice post
 
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