20.4 The pH of weak acids

• The equilibrium concentration of H+

  • dissociation of H+ and A- ions in equal amounts
  • at equilibrium, [H+] = [A-]
• The equilibrium concentration of HA

  • only a very small proportion of HA dissociate
  • [HA] at equilibrium = [HA] undissociated
• With the two assumptions

  • Ka = [H+]^2/[HA]
• [H+] = root(Ka x [HA])
• Approximation for equilibrium [H+] ~ [A-]

  • this breaks down for very weak acids or very dilute solutions
• Approximation for [HA] equilibrium ~ [HA] undissociated

  • this breaks down for stronger acids and concentrated solutions