1
$\begingroup$

In the derivation of $K_\mathrm{w}$, the ionic product of water, it is stated that

$$[\ce{H+}][\ce{OH-}] = K_\mathrm{w}$$

and substituting $[\ce{H+}] = [\ce{OH-}] = 10^{-7}$ we can find that $K_\mathrm{w} = 10^{-14}$.

How do we know, though, that $[\ce{H+}] = 10^{-7}$? And why is $[\ce{H+}] = [\ce{OH-}]$?

|improve this question
$\endgroup$
4
$\begingroup$

Why is $[\ce{H+}] = [\ce{OH-}]$?

Because the water is electrically neutral.

In general, for all solutions, the sum of the concentration of each ion, multiplied by the ion's respective charge, must be zero.

How do we know $[\ce{H+}] = 10^{-7}$?

This is found by experimentally by determining $K_\mathrm{w}$ and substituting, or by measuring the pH.

For details see: Bandura, A. V.; Lvov, S. N. The Ionization Constant of Water over Wide Ranges of Temperature and Density. J. Phys. Chem. Ref. Data 2006, 35 (1), 15–30. DOI: 10.1063/1.1928231.

|improve this answer
$\endgroup$
  • $\begingroup$ Thanks ! But How do we know concentration of H+ is 10^-7 while deriving Kw ? $\endgroup$ – Dimenein Dec 9 '15 at 17:24
  • 3
    $\begingroup$ $K_w$ is measured. It does depend on temperature. pH of pure water is 7.47 at 0 degrees C and 6.14 at 100 degrees C. chemguide.co.uk/physical/acidbaseeqia/kw.html $\endgroup$ – MaxW Dec 9 '15 at 17:26

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.