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• NCEA on TKI
• NZQA website

Chemistry 3.7 Describe properties of aqueous systems

Subject content

In the assessment of this achievement standard:

• Aqueous systems will be limited to those systems involving proton transfer and those involving a sparingly soluble ionic solid.
• The properties of aqueous systems are related to the nature and the concentration of the species present in the solution. The description, explanation and application, or discussion of these properties may be qualitative and/or quantitative.
• Qualitative discussion may involve:
  • correlation between acid or base strength, K_a and pH
  • relative equilibrium concentrations of dissolved species
  • variability in solubility of a sparingly soluble salt due to:
    • the formation of a complex ion
    • the addition of a common ion, or
    • the reaction of a basic anion with added acid
  • features of titration curves including:
    • buffer region
    • equivalence point and
    • selection of indicator (note that titrations of weak acids with weak bases are excluded)
  • the nature of buffer solutions.
• Quantitative evidence may include calculations involving:
  • Ka, Kw, and pH limited to:
    • solutions of bases, monoprotic acids, and buffers
    • those in which the extent of reaction is small so that the equilibrium concentration of a dissolved weak acid can be approximated by the initial concentration, i.e. [HA] = c(HA)
    • pH at a particular point in a titration
  • Ks and solubility limited to:
    • AB, A2B, and AB2 type solids where neither of the ions A or B react further with water
    • calculating the concentration of one ion given the other
    • calculating the solubility in water and in solutions already containing one of the ions A or B (a common ion)
    • predicting precipitation or dissolution.

Some good resources can be found at the following websites:

• Acid–Base Chemistry
• Auckland University's Bestchoice For Chemistry. Select Course 13, Option Acid-base. This is a free site, but note that there is a requirement that you login.
• Chemistry Hypermedia Project – General Chemistry Cross-Index. Revise your understanding of chemical equilibrium and try the equilibrium practice problems.
• Acids and Bases. The topics include an interactive and animated revision of acid and bases, neutralisation and titration, with some calculations.

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