CIE AS Chemistry (9701) exams from 2022

Revision Notes

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

The pH scale, IGCSE & GCSE Chemistry revision notes

The pH scale showing acidity, neutrality and alkalinity

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:

The pH Scale equation

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:

 

Equilibria pH Formula, downloadable AS & A Level Chemistry revision notes

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
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