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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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