Homework Help: Titration question on unkown pH

1. May 27, 2012

sciencestupid

1. The problem statement, all variables and given/known data
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

2. Relevant equations

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

3. The attempt at a solution
literally no idea where to begin

2. May 27, 2012

sjb-2812

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

3. May 27, 2012

sciencestupid

that it dissociates completely in solution

4. May 27, 2012

sjb-2812

OK, how many moles of acid do you have?

5. May 27, 2012

sciencestupid

.036 moles

6. May 27, 2012

sjb-2812

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

7. May 27, 2012

sciencestupid

i dont know... do you?

8. May 27, 2012

sjb-2812

What does triprotic mean?

9. May 27, 2012

sciencestupid

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

10. May 27, 2012

sjb-2812

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

11. May 27, 2012

sciencestupid

.012 or .108??

12. May 27, 2012

sjb-2812

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

13. May 27, 2012

sciencestupid

12? so .012?

14. May 27, 2012

sjb-2812

How did you get 12 to my question?

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

15. May 27, 2012

sciencestupid

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

16. May 27, 2012

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?

17. May 27, 2012

sciencestupid

ya - [log concentration]

18. May 27, 2012

Staff: Mentor

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

19. May 27, 2012

sciencestupid

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

20. May 27, 2012

sjb-2812

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

Last edited: May 27, 2012