# 1.7.10 pH Scale

### The pH Scale

• The pH scale is a numerical scale that shows how acidic or alkaline a solution is
• The values on the pH scale go from 1-14 (extremely acidic substances have values of below 1)
• All acids have pH values of below 7, all alkalis have pH values above 7
• The lower the pH then the more acidic the solution is
• The higher the pH then the more alkaline the solution is

#### pH of water

• An equilibrium exists in water where few water molecules dissociate into proton and hydroxide ions

H2O(l) ⇌ H+(aq) + OH(aq)

• The equilibrium constant for this reaction is: Kc x [H2O] = [H+] [OH]

• Since the concentration the H+ and OH ions is very small, the concentration of water is considered to be a constant, such that the expression can be rewritten as:

Kw = [H+] [OH]

Where Kw (ionic product of water)      =         Kc x [H2O]

=         10-14 mol2 dm-3 at 298K

• Water at 298K has equal amounts of OH and H+ ions with concentrations of 10-7 mol dm-3
• To calculate the pH of water, the following formula should be used: pH       = -log (10-7)

= 7

• Thus, water has a pH of 7

#### pH of acids

• Acidic solutions (strong or weak) always have more H+ than OH ions
• Since the concentration of H+ is always greater than the concentration of OH ions, [H+] is always greater than 10-7 mol dm-3
• Using the pH formula, this means that the pH of acidic solutions is always below 7
• The higher the [H+] of the acid, the lower the pH

#### pH of bases

• Basic solutions (strong or weak) always have more H+ than OH ions
• Since the concentration of OH is always greater than the concentration of H+ ions, [H+] is always smaller than 10-7 mol dm-3
• Using the pH formula, this means that the pH of basic solutions is always above 7
• The higher the [OH] of the base, the higher the pH
Close Close