## Weak acids

Philip Lee 1L
Posts: 30
Joined: Fri Sep 28, 2018 12:21 am

### Weak acids

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
Posts: 61
Joined: Fri Sep 28, 2018 12:19 am
Been upvoted: 1 time

### Re: Weak acids

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.

Posts: 96
Joined: Fri Sep 28, 2018 12:16 am
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### Re: Weak acids

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.