pHpH is a measure of hydrogen ion concentration in aqueous solution. In water, we know that it undergoes the so-called "autoprotolysis reaction"autoprotolysis reaction:
2H_2O(l) rightleftharpoonsH_3O^(+) + HO^-2H2O(l)⇌H3O++HO−
And at 298*K298⋅K, we know that K_w=[H_3O^+][HO^-]=10^(-14)Kw=[H3O+][HO−]=10−14.
And this is an equation, that we can divide, multiply, add to, subtract from....etc. PROVIDED that we do it to both sides of the equality.
One thing we can do is to take log_10log10 of both sides....
And so log_10K_w=log_10[H_3O^+]+log_10[HO^-]log10Kw=log10[H3O+]+log10[HO−]
On rearrangement......
underbrace(-log_10[H_3O^+])_(pH)-underbrace(log_10[HO^-])_(pOH)=-log_10K_w
But -log_10K_w=-log_10(10^-14)=-(-14), and thus, finally,
14=pH+pOH in aqueous solution under standard conditions. How do you think K_w would evolve under NON-STANDARD conditions, i.e. T=100 ""^@C. Remember that the autoprotolysis reaction is bond-breaking.
