# Can Acids have pH of more than 7? [duplicate]

Consider a solution of $$\text{HCl}$$ with a concentration of $$10^{-10}$$ $$\text{M}$$. Now, if I find it's pH: \begin{aligned}\text{pH} &= -\log([\text{H}^+])\\ \text{pH} &= -\log(10^{-10}) = 10\end{aligned} At room temperature, $$\text{pH} + \text{pOH} = 14 \implies \text{pOH}$$ of the given acid is $$4$$. This means that the $$[\text{OH}^-]$$ ions is $$10^{-4}$$ $$\text{M}$$. But from where do these $$\text{OH}^-$$ ions come?

• At this dilution rates you can no longer get away with using concentration instead of activity, and you have to calculate $\mathrm{pH}$ as $\mathrm{pH} = -\log a(\ce{H+}).$ Note that $\mathrm{pH}$ also heavily relies on solvent/medium. – andselisk May 31 '19 at 7:54
• I object. At dilutions this great, you can more than ever rely on using concentration instead of activity. Now to the point. What is the pH of pure water? – Ivan Neretin May 31 '19 at 8:24
• @andselisk right answer, but to another question ;-) – Karl May 31 '19 at 8:24
• In one word: Autoprotolyis – Karl May 31 '19 at 8:26
• In acid base reactions in water there is always the reaction $\ce{H2O <=> H^+ + OH^-}$ occurring and this cannot be ignored. The equilibrium constant $K_w=10^{14}$ so the $\ce{[H^+]}=10^{-7}$ from this reaction far exceeds that from $10^{-10}$ molar acid. see answer chemistry.stackexchange.com/questions/100346/… – porphyrin May 31 '19 at 10:32