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

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SUMMARY

The pH of a strong triprotic acid before titration can be determined by calculating the concentration of protons released upon complete dissociation. In this discussion, a 24 ml solution of 1.5 M unknown strong triprotic acid yields 0.036 moles of acid, resulting in 0.108 moles of protons. The concentration of protons is calculated using the formula [H+] = moles/volume, leading to a pH calculation that initially resulted in an incorrect value due to volume miscalculations. The correct approach emphasizes that strong acids dissociate completely, and the pH should be a positive value.

PREREQUISITES
  • Understanding of strong acids and their complete dissociation in solution
  • Knowledge of triprotic acids and their molecular structure (e.g., H3A)
  • Familiarity with pH calculations and the formula pH = -log[H+]
  • Basic skills in stoichiometry for calculating moles and concentrations
NEXT STEPS
  • Research the properties and examples of strong triprotic acids
  • Learn about pH calculations for polyprotic acids
  • Study the concept of negative pH and its implications in chemistry
  • Explore titration techniques and calculations for strong acid-base reactions
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Chemistry students, educators, and anyone interested in acid-base chemistry, particularly those studying titration and pH calculations of 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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