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When we calculate the concentration of water at $ 25~^\circ\rm C $, why don't we assume dissociation of water at this temperature?

$$\ce{H2O <=> H+ + OH-}$$

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    $\begingroup$ I didn't get it. What should we assume, and how? $\endgroup$ – Ivan Neretin May 6 '16 at 18:43
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    $\begingroup$ Because the number of moles of dissociated water is so slight compared to the number of moles of undissociated water, it makes no practical difference to our calculations to ignore the dissociated fraction. $\endgroup$ – hBy2Py May 6 '16 at 18:46
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    $\begingroup$ Compare the free energy to perform the dissociation with the thermal energy $kT$. $\endgroup$ – Jon Custer May 6 '16 at 18:51
  • $\begingroup$ If you take a look at the equilibrium constant for your reaction, you'll see why. @Ivan please don't forget about bad titles, for the sake of Mart-the-mod's heart. :) $\endgroup$ – M.A.R. May 6 '16 at 19:18
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The equilibrium constant of the reaction is
$$\ce{H2O <=> H+ + OH-}$$ is 10^-14 and hence only 1 atom of water in 10^7 dissociates which really is negligible.

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