Power of Hydrogen

pH scale

[spaced-paragraph]
+ 0
+ Acid
+ Yea yea
0 Acid

7 Neutral

14 Base

0 7 14 Acid Neutral Base

The scale is logarithmic which means tht an increment of 1 pH point is the equivalent of a decrease in the concentration of hydronium ions by a factor of 10

pH

4 - 3

7 - 5

5 - 1

In general: pH = -log[H${}_3$O${}^{+}$] [H${}_3$O${}^{+}$] =

A similar scale exists for hydroxide ions

pOH = -log[OH${}^{-}$] and [OH${}^{-}$] = 10 ^-$$^p$$^O$$^H

There is a property of water which accounts for this it is called the Autoionization of Water.

If we have a pure sample of water then it’s pH is 7

$\therefore$ [H${}_3$O${}^{+}$] = 10^-$$^p$$^H = 10^-$$^7

If it’s pure water where are we getting hydrogen from?

H${}_2$O + H${}_2$O -> H${}_3$O${}^{+}$ + OH${}^{-}$

Some of the water molecules ionize to give us low concentrations of both H${}_3$O${}^{+}$ and OH${}^{-}$ ions. As a result, for pure water, these two concentration are equal at 1 * 10^-$$^7 mol / L

As the concentration of one increases, the concentration of the other has to decrease

As a result of the solubility product (grade 12) [H${}_3$O${}^{+}$][OH${}^{-}$] = 10^-$$^1$$^4 for any aqueous solution

$\therefore$ pH + pOH = 14

From this we can always determine one from the other