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

[sciencestupid]

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

 

[sjb-2812]

What does it mean for an acid to be "strong"?

 

[sciencestupid]

that it dissociates completely in solution

 

[sjb-2812]

OK, how many moles of acid do you have?

 

[sciencestupid]

.036 moles

 

[sjb-2812]

And so how many moles of protons, when it dissociates?

 

[sciencestupid]

i don't know... do you?

 

[sjb-2812]

What does triprotic mean?

 

[sciencestupid]

that the formula is H3___ (3 being a subscript) so its got 3 hydrogen

 

[sjb-2812]

OK, so if there are 0.036 moles of acid, how many hydrogens?

 

[sciencestupid]

.012 or .108??

 

[sjb-2812]

If a cake needs 3 eggs, and I have 4 cakes, how many eggs do I need?

 

[sciencestupid]

12? so .012?

 

[sjb-2812]

How did you get 12 to my question?

Consider that you have 0.036 cakes, each containing 3 eggs...

 

[sciencestupid]

so .108 i did .012 because i thought you were saying 12 egga for 4 cakes

 

[sjb-2812]

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?

 

[sciencestupid]

ya - [log concentration]

 

[Borek]

And what is the concentration? (given that you know number of moles and volume)

 

[sciencestupid]

1.38, but that's too high the negative log of that is negative itself

 

[sjb-2812]

How have you got 1.38? Also, negative pH is not a problem either.

 

[sciencestupid]

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.

 

[sjb-2812]

Why do you think it's too high?

 

[sciencestupid]

cause you get a negative pH

 

[sjb-2812]

And that's a problem because?

 

[sciencestupid]

there is no such thing as a negative pH...

 

[sjb-2812]

http://antoine.frostburg.edu/chem/senese/101/acidbase/faq/negative-pH.shtml ?

 

[Alistair Over]

i have the sme question! nice post