 # The pH Scale

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## The pH Scale

#### How to calculate pH

The concentration of oxonium ions in solution can be expressed in terms of the pH scale.

The pH of a solution is the logarithm to base 10 of the reciprocal of the numerical value of the oxonium ion concentration:

pH = lg 1/H30+

= -lg [H30+]

= -lg [H+]

The pH of a neutral solution can be calculated directly from the ionic product of water.

Kw = [H3O+] = [OH-] OR [H+] = [OH-]

[H+] = 10-7

pH = 7 for neutral solutions.

Acidic solutions have a pH less than 7 at 25oC.

#### pH values of bases

The concentration of hydroxide ions in a solution can be expressed in terms of pOH. This is given by: pOH = -lg[OH-]

It is possible to write an expression relating pH and pOH as pKw = pH + pOH

At 25oC: pH + pOH = 14

#### Calculation examples

Calculate the pH value of 0.001 moldm-3 solution of NaOH at 25oC.

[OH-] = 10-3

Therefore: p[OH-] = 3

If pH + 3 = 14, then pH = 11

Calculate the pH of a solution of 0.1M hydrochloric acid at 25oC:

[H+] = 10-1 moldm-3

pH = -log10-1 = 1

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