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The Concept of pH

In order to describe the acidity of a solution, scientists have devised the pH scale. The pH is defined as the negative log of the hydrogen-ion concentration.
pH = – log [ H+]


Example 9-1

What is the pH of a solution having a 1 x 10–2M hydrogen-ion concentration?



The formula for pH is pH = – log [ H+ ]. Substituting in this formula,


pH = – log (1 x 10–2) = 2


The pH of this solution is 2.

We can also find pH if we know the hydroxide-ion concentration.


pOH = – log [OH]

Since pH + pOH = 14, if we know the pOH, the pH can be easily calculated or vice versa.


Example 9-2

Find the pH of solution X, if it has a hydroxide-ion concentration of 1 x 10– 8M.



Since the hydroxide-ion concentration is 1 x 10– 8M, the pOH is 8. Since we know that pH + pOH is 14,


pH = 14 – 8 = 6


The pH value of solution Xis 6.

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