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Autoprotolysis

Posted: Thu Jan 24, 2019 4:11 pm
by Emmaraf 1K
At what concentration of equilibrium hydronium ions must we consider the autoprotolysis of water as being an important factor in pH calculation. Is it 10^-6 or 10^-7?

Re: Autoprotolysis

Posted: Thu Jan 24, 2019 4:35 pm
by Ethan Baurle 1A
I believe it is any value higher than 1 x 10^-7 that will make a difference in the PH.

Re: Autoprotolysis

Posted: Fri Jan 25, 2019 1:49 pm
by monikac4k
If a weak acid deprotonates and releases less than 10^-7 M of hydronium, then you would consider autoprotolysis. This is because if you ignore autolysis and calculate pH of an acid that creates 10^-9 M hydronium, then the pH would be basic. This would not make sense since there is an acid in the solution. To fix this error, you consider the hydronium ions from autoprotolysis and you should get a more acidic pH.

Re: Autoprotolysis

Posted: Fri Jan 25, 2019 2:26 pm
by Dayna Pham 1I
monikac4k wrote:If a weak acid deprotonates and releases less than 10^-7 M of hydronium, then you would consider autoprotolysis. This is because if you ignore autolysis and calculate pH of an acid that creates 10^-9 M hydronium, then the pH would be basic. This would not make sense since there is an acid in the solution. To fix this error, you consider the hydronium ions from autoprotolysis and you should get a more acidic pH.


I agree with this. With [H3O+] values smaller than 10^-7 M, when you take the pH, it seems to be basic, but this is not the case. You would have to consider that the H3O+ is actually in water and surrounded by water, so you would add that small [H3O+] value, let's say 10^-10, to 10^-7 (the pH of the water around it), and determine that the solution is actually neutral.

This is because 10^-10 + 10^-7 is approximately 10^-7.