Ionization of Water

We learned in my previous post the different weak electrolytes and strong electrolytes.  Strong electrolytes are 100% dissociated or ionized in water while weak electrolytes only few % are ionized in water.  Example is the salt in water solution, using electrical conductivity apparatus we can tell that its a strong electrolyte because it can produce very bright light while acetic acid on the other hand is a weak electrolyte, it only produce dim light in electrical conductivity apparatus.  Water on the other hand is an example of  weak electrolyte, lets see what happens in when water is combined with another water molecule:

Water molecule reacts with another water molecule forming hydronium ion and hydroxide ion.  The double arrow indicates that a reversible reaction occur, hydronium ion and hydroxide ion can also combine forming water molecules.  This is called autoprotolysis (or autoionization)  of water.  The bond breaking between water molecule and ions combining forming water molecules continues until the same rate happens, and this is called state of chemical equilibrium.

We can write simply the ionization of water molecules as follows:

The concentration of hydrogen ion in pure water is  mole per liter at . 

Since the ionization of water gives one H+ and OH-  , therefore the ionization of hyroxide ion is also  mole per liter at the same temperature.  The product of the concentration of the H+ and the OH- is called the equilibrium constant of water (symbol Kw).  Chemist use bracket [ ] to represents the concentration, therefore [H+] is the concentration of hydrogen ion and [OH-] is the concentration of hydroxide ion.  Kw of water can be calculated using the formula:

Calculating the Kw of water

Based from the calculation above Kw (equilibrium constant) of water is . Since Kw is constant whenever there is a change in the concentration of hydrogen ion the concentration of hydroxide ion is also changes or vise versa.

Example:

Calculate the hydroxide ion concentration [OH-] given the hydrogen ion concentration [H+]:

Solution:

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