Weak acids

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Philip Lee 1L
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Joined: Fri Sep 28, 2018 12:21 am

Weak acids

Postby Philip Lee 1L » Thu Jan 17, 2019 8:20 pm

If a really really weak acid is added to water and the change in [H3O+] is very small (like 10-9), then we say that the pH is essentially unchanged. 10-7 + 10-9 is really close to 10-7, so we say pH is 7.

If a stronger acid is added to water and the change in [H3O+] is larger, do we need to add it to the original [H3O+] of water? For example, if adding an acid into water changes [H3O+] by 10-6, do we need to say that the new [H3O+] is 10-7 + 10-6?

Anushi Patel 1J
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Re: Weak acids

Postby Anushi Patel 1J » Thu Jan 17, 2019 8:31 pm

I was also wondering this. I think we don't have to, since the change from the acid is substantially larger than the original [H3O+] of water, but I'm not sure.

Venya Vaddi 1L
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Re: Weak acids

Postby Venya Vaddi 1L » Thu Jan 17, 2019 8:31 pm

I think you would just calculate [H3O+] using Ka for the stronger acid because it changes the pH of the solution.

"If a really really weak acid is added to water and the change in [H3O+] is very small (like 10-9), then we say that the pH is essentially unchanged. 10-7 + 10-9 is really close to 10-7, so we say pH is 7."

^For that, essentially the acid that was added was so weak that the [H3O+] was unaffected and thus the solution remains neutral.


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