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What is the relationship between the concentration of hydrogen ions
and pH?
pH is a measure of the number of H+ ions
in water. Pure water (as described above) contains of an equal
number of hydrogen ions and hydroxyl ions (1 x 10-7 moles per liter).
pH can be calculated using the following formula:
pH = - log [H+]
where the brackets [ ] represent the concentration
of hydrogen ions in a liter of water or in a solution containing
water.
Therefore, the pH of pure water can be
calculated as:
pH = - log
[H+] = - log [1 x 10-7] = -[-7] = 7
A solution that has more hydrogen ions per liter
than pure water is referred to as being acidic.
A solution that has fewer hydrogen ions than pure water
is referred to as being basic.
Would a solution that contains 1 x 10-5
moles of hydrogen ions per liter contain more or fewer hydrogen
ions than pure water? (Remember pure water contains 1 x 10-7 moles of hydrogen ions per liter.)
If you answered more, you are correct! The
pH of such a solution could be calculated to be:
pH = - log [H+]
= - log [1 x 10-5] = -[-5] = 5
A solution with a pH less than 7 is called an acidic
solution; therefore, the pH of 5 indicates the solution acts like
an acid.
Would a solution that contains 1 x 10-9
moles of hydrogen ions per liter contain more or fewer hydrogen
ions than pure, deionized water? (Remember pure water contains 1 x 10-7 moles of hydrogen ions per liter.)
If you answered fewer, you are correct! The
pH of such a solution could be calculated to be:
pH = - log [H+]
= - log [1 x 10-9] = -[-9] = 9
A solution with a pH greater than 7 is called a
basic solution; therefore, the pH of 9 indicates the solution acts
like a base.
In summary, pH is normally measured on a scale from
0 to 14, 0 being the most acidic and 14 the most basic. This
scale is called a logarithmic scale. Note that a change of
1 pH unit represent a 10 fold change in hydrogen ion concentration.
A solution that has a pH = 5 has 100 times more hydrogen ions
per liter than a solution with a pH = 7.
Click
here to view a pH scale and find out about acid rain.
 
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