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Towards the end of the lecture today, the professor was talking about what would happen if the concentration was less than 10^-7. Can someone re-explain what he meant by this especially when he brought up using 10^-9?
If the hydronium ion concentration is less than 10^-7, e.g. 10^-9, the solution is considered neutral; a solution producing such a small amount of hydronium ions, which doesn't really affect the pH (hence why it's neutral). Autoprotolysis generates 10^-7 molL^-1 of hydronium ions.
He mentioned this detail in order for you to not be confused when you end up with a pH over 7 (basic) while calculating the pH of a solution mixed with a very very weak acid (that means you did it wrong). When calculating for pH, always remember that the hydronium concentration of the water you begin with is 1.0x10^-7 (while at the right temperature), so adding a very very weak acid would make the overall pH just under 7.
Basically the acid is so weak the overall solution can be considered neutral. Such a small concentration of H3O+ is pretty much not affecting the pH anymore in the grand scheme of things
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