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In lecture, Dr. Lavelle said that if the concentration of an acid introduced to a system is less than 10-9 the pH should be regarded as 7 because the contribution of H30+ ions is so negligible. I'm wondering: when the concentration is between 10-7 and 10-9, should we also neglect it or should we add it to the hydronium concentration from the autoprotolysis of water (1*10-7)?
I believe if we come out with any concentration for H3O+ that is smaller than 10^-7 we should still consider it as slightly acidic but very close to neutral since we know that autoprotolysis generates 10^-7 mol/L of H3O+, so it wouldnt make sense of our H3O+ concentration was less than that. If we do get a value less than 10^-7 then we are basically adding it to the 10^-7 mol/L of H3O+ that we already have which produces a value slightly greater than 10^-7 mol/L of H3O+, and thus a pH value slightly under 7. The amount of H3O+ concentration added is so negligible that we can consider it to be of neutral pH.
To add on, I don't think Dr. Lavelle would give us a borderline problem like that. If he did however, I'm pretty sure you still consider it negligible and add it.
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