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Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: saratan on August 16, 2017, 11:42:33 PM

Title: Finding the pH of an amphiprotic acid (HPO4-2) in .1 M solution.
Post by: saratan on August 16, 2017, 11:42:33 PM

So I was asked to find the pH of .1 molar of this amphiprotic acid, and this is what I came up with:

(https://www.chemicalforums.com/proxy.php?request=http%3A%2F%2Fi.imgur.com%2FFuf8Bxe.jpg&hash=fa5ceeea52983a22137f613a35051d2b6ddb0fdb)

my answer to the question ended up being pH = 10.1 however the correct answer is 9.78, I am trying not to use a well known equation because I am trying to understand this topic on a deeper level. What could I be doing wrong here? Did I choose the wrong equation to work with? or am I not accounting for another source of [H+] or [OH-]?

Title: Re: Finding the pH of an amphiprotic acid (HPO4-2) in .1 M solution.
Post by: Borek on August 17, 2017, 03:27:29 AM

I would love to help, but do you really want me to break my neck?

Title: Re: Finding the pH of an amphiprotic acid (HPO4-2) in .1 M solution.
Post by: XeLa. on August 17, 2017, 10:35:59 PM

Okay, firstly, what can you tell me about the relative strength of each dissociation? Which would you consider negligible, and which contributive to the pH of this 0.1-molar acid?

XeLa

Title: Re: Finding the pH of an amphiprotic acid (HPO4-2) in .1 M solution.
Post by: saratan on August 17, 2017, 11:45:17 PM

So I came to the conclusion that since the Kb was larger than the Ka, it was therefore the predominant reaction it would overrepresent the pH and that the acid reaction of HPO4 2-  to H2PO4 - was not producing enough [H3O+] compared to the basic reaction of HPO4 2- to PO4 3- producing much more [OH-] ions. and so I took that reaction and made a rice table and solved the rest of the problem like a weak base equilibrium pH problem.

Title: Re: Finding the pH of an amphiprotic acid (HPO4-2) in .1 M solution.
Post by: saratan on August 17, 2017, 11:50:52 PM

Hey Borek, sure I am trying to change the orientation of the photo but how can I edit the post so I can modify the picture?

Title: Re: Finding the pH of an amphiprotic acid (HPO4-2) in .1 M solution.
Post by: Borek on August 18, 2017, 03:08:11 AM

Quote from: saratan on August 17, 2017, 11:50:52 PM

Hey Borek, sure I am trying to change the orientation of the photo but how can I edit the post so I can modify the picture?

At the moment original post can be modified only by moderators (it is past the grace period). Replay to the thread with the upright picture and we will do the rest.

Title: Re: Finding the pH of an amphiprotic acid (HPO4-2) in .1 M solution.
Post by: Borek on August 18, 2017, 07:15:18 PM

Just in case it gets lost in the future, as it often happens with images hosted outside:

Title: Re: Finding the pH of an amphiprotic acid (HPO4-2) in .1 M solution.
Post by: Borek on August 18, 2017, 07:17:41 PM

In the case of amphiprotic substances you can't ignore one of the reactions just because:

http://www.chembuddy.com/?left=pH-calculation&right=pH-amphiprotic-salt

Title: Re: Finding the pH of an amphiprotic acid (HPO4-2) in .1 M solution.
Post by: saratan on August 20, 2017, 03:19:41 AM

So what does this mean? I mean logically in the sence of how I can interpret this information so I can understand it, the link was a bit confusing. Im trying to understand this on a deeper level, that is why I am using rice tables and doing it the long way, becuase I dont want to just use an equation that I know I will forget and dont understand its meaning.