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Calculating pH

Sodium Chloride

Understanding pH is essential in chemistry and biology. pH is defined by the following equation,

pH = −log [H+] ,

where [H+] denotes the molar hydrogen ion concentration. Notice that we are required to take the common (base 10) logarithm of the hydrogen ion concentration in order to calculate pH.

Because pH is a measure of hydrogen ion concentration, it is used to quantitatively characterize solutions as acidic, neutral, or basic (alkaline). The typical pH scale runs from 0 - 14. A pH of 7 is neutral, a pH < 7 is called acidic while pH > 7 is called basic.

A note of caution while working with pH.

Remember that pH is calculated on a logarithmic scale, therefore small differences in pH represent much larger differences in hydrogen ion concentration.

For example,

a solution with pH 3 ( i.e. [H+] = 1 × 10 − 3 M )

is ten times more acidic than

a solution with pH 4  (i.e. [H+] = 1 × 10 − 4 M )

In fact, for each unit increase in pH, there is a 10 fold increase in the hydrogen ion concentration.

For more on pH see the Acids and Bases Problem Set of the Chemistry section.

Now that you have been introduced to some pH basics, try the following 6 problems.

Problem 1- Calculate the pH of lemon juice

Problem 2 - Calculate the pH of an unknown solution

Problem 3 - Find the hydrogen ion concentration of an unknown solution

Problem 4 -Determine the hydrogen concentration of blood

Problem 5 - Compare the acidity of two solutions

Problem 6 - The pH range of blood


Next Application: Drug Concentrations


The Biology Project > Biomath > Applications > pH

Credits and Citation

The Biology Project
Department of Biochemistry and Molecular Biophysics
The University of Arizona

December 2005
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