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Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: Kirtha on December 18, 2021, 03:42:23 PM

Title: Help with an exercise pH
Post by: Kirtha on December 18, 2021, 03:42:23 PM
Hi, I have been trying to solve this exercise:

Check that the pH of these two solutions for a concentration of 0.01M is 4.6 and 9.6. Givens: H2P04 pka1=2.15, pka2=7.2 and pka3=12.4
(https://preview.redd.it/hsrzlbtsnc681.png?width=888&format=png&auto=webp&s=977213993193d1d13fc1e9a263da638e58809682)

I managed to prove that the 1st pH is 4.6 but I have no clue how to get the 9.6 one? Is this something anyone can help me to understand how to do it?

Thanks!
Title: Re: Help with an exercise pH
Post by: Borek on December 18, 2021, 03:57:33 PM
I don't see two solutions.
Title: Re: Help with an exercise pH
Post by: Kirtha on December 18, 2021, 04:10:35 PM
Yeah, that confuses me too. But unfortunately. that is everything what the problem provides.
Title: Re: Help with an exercise pH
Post by: Borek on December 18, 2021, 07:07:22 PM
If so, question doesn't make sense.

Also, pH of 4.6 for a 0.01M solution of phosphoric acid (assuming that's what the question intends to ask) is over two units off.
Title: Re: Help with an exercise pH
Post by: Babcock_Hall on December 20, 2021, 08:29:47 AM
I have not checked the numbers (busy morning for me), but 4.6 sounds in the right ballpark for a solution of the mono-anion, H2PO41-.