Answer 1:
The pH scale is related to the concentration of
hydrogen ions in solution. Water can disassociate
into hydrogen and hydroxide ions. Chemically this
can be expressed as:
H_{2}O >
H^{+} + OH^{}
Within a
certain amount of water, molecules are constantly
disassociating into hydrogen and hydroxide ions,
which then may recombine to form water again.
When the rate of disassociation and recombination
are equal, the concentration of hydrogen and
hydroxide ions remains constant, which is called
equilibrium. There is an equation that
predicts
what concentrations of hydrogen and hydroxide ions
can be present in equilibrium.
In
equilibrium, the logarithm of the hydrogen ion
concentration plus the logarithm of the hydroxide
ion concentration is 14. This can be
written:
log[H^{+}] +
log[OH^{}] = 14
or
equivalently:
[H^{+}]*[OH^{}]
= 0.00000000000001
(note that brackets mean
concentration, so [H^{+}] is the
concentration of hydrogen ions in moles per
liter). The pH scale is based on the
concentration of hydrogen ions. The pH is equal
to the logarithm of the hydrogen concentration
multiplied by negative one.
pH = 1 *
log[H^{+}]
If pH is low, the solution
contains more hydrogen ions. If the pH is below 7,
there are more hydrogen ions than hydroxide ions.
Can you find an equation to determine how many
hydroxide ions are in a solution of a given
pH?
(Answer: log[OH^{}] = pH  14 )
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