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When the pH is negative, it just means that it is a very very very strong acid, and the H30+ concentration is extremely high. The pH level will be negative when you have a H30+ concentration greater than 1.0M. The pH range of 0-14 is only in general, there will be acids that can be less than zero pH, and there will be bases that are pH greater than 14.
ASetlur_1G wrote:Why can the pH sometimes be negative? What's going on conceptually?
If the concentration of H3O+ is greater than 1M, the pH will be 0 (if it is equal to 1) or negative. The reason that the scale is 1-14 is because in most systems, the concentration of H3O+ does not exceed 1M. Conversely, if the concentration OH- is larger than 1M, you can get a pH that is higher than 14. It is important to note that even though these situations are possible, they are not probable.
pH is simply -log([H3O+]). When the molarity of H3O+ is higher than 1 M, then the pH will begin to become negative. Acids aren't typically this strong, and there's actually a special category for such strong acids called superacids, but they aren't common.
I remember the book saying that it's possible to have a negative pH or a pH higher than 14, but that they wouldn't ask us questions outside the scope of a pH value from 1-14. Any acid that has a concentration of hydrogen ions with a molarity greater than 1 will likely be calculated to have a negative pH.
Although the concentration of H+ ions cannot become negative, pH can become negative since pH = -log[H+]. Therefore, when the H+ concentration is greater than 1 M, the pH can become negative. However, we don't see negative pHs all that often because most reactions don't have H+ concentrations as high as 1 M.
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