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Chemistry - acids

Sounds like a silly question
but how is the concentration of water relatively constant?
Reply 1
Not quite sure what you're asking here, but perhaps you are wondering about the acid-base equilibrium in water:

H++OHH2O\mathrm{H}^{+}+\mathrm{OH}^{-}\leftrightharpoons \mathrm{H}_{2}\mathrm{O}

and what happens when this is shifted away from usual conditions in pure water at pH = 7 by the addition of an acid or base in solution? Let's assume that's the case, as it's an interesting question.

Since the water has molar mass 18 g/mol and density around 1 g/cc at s.t.p., then its molar volume is 18 cc/mol, or 0.018 dm3/mol. This means that the "concentration" of water in bulk water is about 1/0.018 = 55.6 M.

So, in order to have a significant effect on the concentration of water, the pH would have to be below around -1, at which point the pH scale becomes dubious anyway. For most normal pH ranges, the effect on water concentration of shifting the acid-base equilibrium is negligible.
Reply 2
The question is not silly at all, and it's an interesting aspect of the properties of water. But, I'm not exactly sure what you mean exactly by concentration in which context.

The concentration of water is relatively constant because water is a very stable and abundant substance on Earth, and it's involved in a variety of natural processes that help maintain its concentration.

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