= 10−14 M^2 at 25°CFurthermore, at constant temperature, regardless of whether more [H+] or more
[OH−] is added to the solution, the Kw, or [H+] × [OH−], of any aqueous solution
is equal to 10−14 M^2 at 25°C. That’s because the only way to change the value of
an equilibrium constant is to change the temperature of the solution.
What Is pH?
The [H+] of pure water at 25°C is 10−7 M. Many people find working with
exponents, especially negative exponents, a bit scary. Luckily for you, the
tradition of pH was conceived and created; pH and [H+] have the following
relationship:
If [H+] = 10−7 Mthen pH = −log (10−7) = 7Note that “p” is the abbreviation for the numerical operation of −log; the −log is
taken from whatever number follows the p. For example, given that
Kw = [H+][OH−] = 10−14 M^2taking the p of every term gives
pKw = p[H+] + p[OH−] = 14Notice that we don’t include units here, so after taking the −log, the resulting
number has no units.
Doing Log 10 in Your Head
For some people, having to figure out a base ten logarithm (log 10 ) is just as
daunting as working with exponents. If you are one of these people, try the
following trick:
Look at the number, and ask the question: “What’s this number’s