pH Scale (Cambridge (CIE) A Level Chemistry): Revision Note

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 below 7

  • All alkalis have pH values above 7

• The lower the pH, the more acidic the solution is

• The higher the pH, the more alkaline the solution is

The pH scale

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 expression for this reaction is:

K_c = $\frac{\left[H^{+}\right]\left[{\mathrm{OH}}^{-}\right]}{\left[H_{2}O\right]}$

• The equilibrium constant expression can be rearranged to:

K_c x [H₂O] = [H⁺] [OH^–]

• Since the concentration of the H⁺ and OH^- ions is very small, the concentration of water is considered to be a constant

  • So, the expression can be rewritten as:

K_w = [H⁺] [OH^–]

• Where K_w (ionic product of water) = K_c 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 [H⁺ (aq)]

• Where [H⁺ (aq)] is the concentration of H⁺ / H₃O⁺ ions

• So, the calculation is:

  • 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 OH^- than H⁺ 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