# how to solve problems in ionic equilibrium

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Perhaps the most important ionic equilibrium is the ratio of hydrogen ions to hydroxide ions in pure water. This can be used to deduce the dissociation constant of water.

It is known, from measurements, that the equilibrium constant for water @STP (standard temperature = zero celcius, standard pressure = 101.3 kilopascals) is ten to the power of minus fourteen.

Expressed mathematically:

[H+] x [OH-] = 10^-14

Where [H+] is the concentration of hydrogen ions in moles per litre, and [OH-] is the concentration of hydroxide ions in moles per litre.

This simple formula is the basis for the solving of a multitude of problems in preparation for university entrance to do chem.

One example:

What is the pH of a 0.1 mole per litre (0.1M) solution of sodium hydroxide solution?

NaOH is a strong base and it is assumed that it dissociates in water completely so that the concentration of OH- can be assumed to be 0.1M ( = 0.1 moles per litre OH minus ions) = 10^-1M

This affects the above mathematical equation like this:

[H+] x [10^-1] = 10^-14

Rearranging for [H+]:

[H+] = (10^-14)/[10^-1] = 10^-13.

pH = -log[H+] = 13 the correct value for a dilute alkali solution.

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