Weak acids

[Philip Lee 1L]

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]

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]

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.