While calculating $K_i$ i.e. ionization constant of water, why do we put $[H_2O]=55.5M$ in the expression below:-

$K_i= \dfrac{\ce{[H^+][OH^-]}}{\ce{[H_2O]}}$

$$\ce{H_2O <=> H^+ + OH-}$$

although we know that activity of pure liquids is 1.

So going by that should we not put $[H_2O]=1$ in the expression for $K_i$


  • $\begingroup$ I understand why the concentration of pure water is 55.55 mol/L but my doubt is, in the expression for $K_i$ we use activities and not concentrations of substances and hence shouldn't we be using $[H_2O]=1$ as activity of pure liquids(and pure solids) is taken as 1 $\endgroup$
    – Alok
    Aug 8 at 14:47
  • $\begingroup$ [H2O] is concentration, not activity. $\endgroup$
    – Poutnik
    Aug 8 at 14:53
  • $\begingroup$ As per the law of Mass action (using which we derive the expression for Equilibrium constant), it is more precise to use Activities instead of concentrations. The same can be read from the link below chem.libretexts.org/Bookshelves/… So I still have the same doubt. $\endgroup$
    – Alok
    Aug 8 at 15:11
  • $\begingroup$ Then do not use concentration instead of activity. For compound $\ce{A}$, concentration is expressed as $c_\mathrm{A}$ or $\ce{[A]}$. Activity is then $a_\mathrm{A}$. Remember that activity for water is defined differently than for ions. $\endgroup$
    – Poutnik
    Aug 8 at 15:44
  • 1
    $\begingroup$ As you have said yourself $a_{\ce{H2O}}=1$ for pure water or diluted enough solution, by definition. As in contrary to concentration, it depends on convention what state is taken to have $a=1$. $\endgroup$
    – Poutnik
    Aug 8 at 17:40

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