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The pH of a solution is defined using logarithms as
$$
pH = -\log_{10}[H^+],
$$
where $[H^+]$ is the concentration of $H^+$ ions in mol/l of the solution.
Imagine that this equation works for any concentration of $H^+$, and imagine that scientists have created conditions in a 100ml sample of pure water in which only one molecule of water has dissociated into $H^+$ and $OH^{-}$ (such conditions are, or course, science fiction!)
What would the pH of this fluid be?
Next imagine that conditions arise in which every single molecule of water dissociates.
What would the pH be now? What assumptions do you make in the calculation?
Under real-world situations, what volume of water would we expect to contain exactly one $H^+$ ion?