February 28, 2020, 08:24:52 PM
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Topic: How come polyprotic acids are less dissolved at lower pH values?  (Read 170 times)

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Offline smoky

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How come polyprotic acids are less dissolved at lower pH values?
« on: February 10, 2020, 08:17:04 PM »
It seemed to me the more a polyprotic acid dissolved, the higher the amount of H+ ions would be in the solution, and therefore the pH value would be lower. However, in my textbook it looks like it's saying otherwise.

It shows the fractions of the species (undissolved acid, partially dissolved, and completely dissolved) as a function of pH, with the lowest pH values having the highest amounts of undissolved. What's going on here? Does it have something to do with the addition of another substance that contains common ions?

Any help would be appreciated!

Offline Borek

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Re: How come polyprotic acids are less dissolved at lower pH values?
« Reply #1 on: February 11, 2020, 03:26:00 AM »
It seemed to me the more a polyprotic acid dissolved, the higher the amount of H+ ions would be in the solution, and therefore the pH value would be lower.

I guess you mean dissociated, not dissolved. If so - that's how it is.

Quote
However, in my textbook it looks like it's saying otherwise.

Nope :)

Quote
It shows the fractions of the species (undissolved acid, partially dissolved, and completely dissolved) as a function of pH, with the lowest pH values having the highest amounts of undissolved.

Quite correctly.

There is no contradiction here - increase in the acid concentration makes up for the decreasing fraction of the dissociated acid, so the concentration of H+ still grows (and pH goes down). Yes, it can be to some extent explained in terms of Le Chatelier's principle.
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Offline smoky

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Re: How come polyprotic acids are less dissolved at lower pH values?
« Reply #2 on: February 11, 2020, 04:32:22 PM »
It seemed to me the more a polyprotic acid dissolved, the higher the amount of H+ ions would be in the solution, and therefore the pH value would be lower.

I guess you mean dissociated, not dissolved. If so - that's how it is.

Quote
However, in my textbook it looks like it's saying otherwise.

Nope :)

Quote
It shows the fractions of the species (undissolved acid, partially dissolved, and completely dissolved) as a function of pH, with the lowest pH values having the highest amounts of undissolved.

Quite correctly.

There is no contradiction here - increase in the acid concentration makes up for the decreasing fraction of the dissociated acid, so the concentration of H+ still grows (and pH goes down). Yes, it can be to some extent explained in terms of Le Chatelier's principle.

Alright thanks, got it now!

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