Acid, Bases and Buffers

• Monobasic

  1 mole of acid dissociates to form 1 mole of H+ ions
• Dibasic

  1 mole of acid dissociate to form 2 moles of H+ ions
• Tribasic

  1 mole of acid dissociates to form 3 moles H+ ions
• Buffer Solution

  A system that minimises pH changes on addition of small amounts of an acid or a base.
• Why is the pH scale used for aqueous solutions?


  The hydrogen ion concentrations in aqueous solutions of acids and alkalis have a very large range of values.

  The negative logarithm to the base 10 of the hydrogen ion concentrations in aqueous solutions produce a more manageable scale.
• How to measure pH?


  Calibrate a pH probe attached to a pH meter with buffer solutions of known pH.

  Measure the pH of the solution with a pH probe attached to a pH meter, with the solution a the pH probe immersed in the solution.

  Set to 25.0 degrees celcius while the measurements are taken.
• How is a buffer solution prepared?

  • By mixing a weak acid and one of its salts. Salt of weak acid provides the conjugate base component of the weka acid buffer solution.
  • By partial neutralisation of a weak acid
  • Aqueous solution of an alkali added to an axcess of a weak acid. The weak acid is partially neutralised by the alkali, forming the conjugate base. Some of the weak acid is left over - unreacted. Resulting solution has a mixture of salt and unreacted weak acid.
• Equivalence point

  The volume of one solution that exactly reacts with the volume of the second.

  • Centre of the vertical section of a pH titration curve.
• End point

  Point for which an indicator has equal concentrations of its weak acid and conjugate base.

  Indicator colour will be in between its two extremes. (Changes colour)
• Strong acid

  An acid that dissociates completely in solution
• Weak acid.

  An acid that only partially dissociates in solution.
• Acid

  Species that releases H+ ions in aqueous solutions.
• Alkali

  A type of base that dissolves in water to release hydroxide ions.
• Brønsted-Lowry acid

  Species that donates a proton H+
• Brønsted-Lowry base

  A proton acceptor
• Base

  A compound that neutralises an acid to form a salt.