While calculating pH of $10^{-8}$M HCl, we take into account the $\ce{H^+}$ ions released by water. But water also releases $\ce{OH-}$ions.So she shouldn't they neutralise the $\ce{H+}$ ions?
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1$\begingroup$ There is a worked answer here chemistry.stackexchange.com/questions/100346/… . and a more general one here chemistry.stackexchange.com/questions/60068/… $\endgroup$– porphyrinJan 27, 2019 at 14:29
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2 Answers
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Yes, water will release H+ and OH- ions and this is known as "self-ionization of water" and this is where the maximum pH in water (i.e. 14) comes from. But you should notice that one molecule of water will always give the same amount of H+ than OH- as H2O -> H+ + OH-
Whenever you add a source of H+, like HCl, then only the concentration of H+ will increase which causes an imbalance compared to pure water and this is where pH comes from. And the opposite is true for, e.g. KOH, which will release only OH- ions and no H+.
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Yes, of course! H+ ions neutralize OH- ions, but they do so at a certain rate. At any given time, there are other, different water molecules generating the exact amount of ions being consumed.
This happens because the system reaches equilibrium, the condition that you assume when calculating pH. The conclusion is that while the H+ ions are constantly changing into water and viceversa, the concentration is always the same.
