Stoichiometric Calculations

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Stoichiometric Calculations

• The workhouse of the analyst

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Review of the Fundamentals

• Molarity (M) moles/Liter
• Molality (m) moles/Kg

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Normality (N)

• The normality of a solution is simply a multiple
  of the molarity of the solution.
• Generally, the normality of a solution is just
  one, two or three times the molarity.
• In rare cases it can be four, five, six or even
  seven times as much.
• The symbol for normality is N.

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• CaCl2? Ca2 2Cl-
• 1 M CaCl2 2 N CaCl2
• 2.4 M CaCl2 4.8 N CaCl2
• Note that calcium chloride has two moles of
  chloride ions for every mole of CaCl2.
• Because of that, the multiplying factor for
  calcium chloride is two.

• AlCl3 ?Al3 3Cl-
• 1 M AlCl3 3 N AlCl3
• 2.4 M AlCl3 7.2 N AlCl3
• Aluminum chloride the multiplying factor is
  three.

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Standard Solution

• A standard solution is a solution whose
  concentration is accurately known.
• Primary Standard A primary standard is a highly
  purified compound that serves as a reference
  material in all volumetric and mass titrimetric
  methods.
• Secondary Standard A second standard is a
  solution of known concentration by titration with
  a solution of primary standard.

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Preparing Standard Acid Solutions

• Hydrochloric acid is widely used for titration of
  bases.
• Dilute solutions of HCl are stable indefinitely
  and do not cause troublesome precipitation
  reactions with most cations.
• 0.1 M solutions of HCl can be boiled for as long
  as one hour without loss acid.
• 0.5 M solutions of HCl can be boiled for at least
  10 min without significant loss.

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• Solutions of perchloric acid and sulfuric acid
  are also stable and are useful for titration
  where chloride ion tnterferes by forming
  precipitates.
• Solutions of nitric acid are seldom encountered
  because of their oxidizing properies.

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Standardizing Acids

• Sodium Carbonate
• 2NaHCO3(s)?Na2CO3(s)H2O(g)CO2(g)
• Heat purified sodium hydrogen carbonate
• between 270 and 300oC.
• The point at pH3.8 is used for standardization.
• An even sharper end point can be achieved by
  boiling the solution briefly to eliminate the
  reaction products, carbonic acid and carbon
  dioxide. The titration is completed after the
  solution has cooled.

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Preparing Standard Base Solutions

• Sodium hydroxide is the most common base for
  preparing standard solutions.
• The effect of CO2 CO22OH-?CO3-2H2O
• If the acidic solution
• CO3-2H3O?H2CO32H2O
• In the basic solution
• CO3-2H3O?HCO3-H2O (carbonate error)

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Preparing carbonate-free NaOH solution

• Approximate 50 aqueous solution of NaOH
• Dilute to desired concentration
• Distilled water should be bolied to eliminate the
  gas.
• The water is allowed to cool to room temperature.
• NaOH solution will decrease slowly by 0.1-0.3
  per week.

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Requirements for a Primary Standard

• High purity (established methods for confirming
  purity should be available)
• Atmospheric stability
• Absence of hydrate water so that the composition
  of the solid does not change with variations in
  relative humidity
• Ready availability at modest cost
• Reasonable solubility in the titration medium
• Reasonably large molar mass so that the relative
  error associated with weighing the standard is
  minimized.

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Some primary standards for titration of bases

• 1. Potassium Hydrogen Phthalate KHC8H4O4 (KHP)
  mw 204.23 g/mol
• 2. Benzoic Acid its solubility in water is
  limited, this reagent is ordinarily dissolved in
  ethanol prior to dilution water.
• 3. Potassium Hydrogen Iodate KH(IO3)2 is an
  excellent primary standard with a high molecular
  mass per mole of protons. mw 389.92 g/mol

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Some primary standards for titration of acids

• sodium carbonate Na2CO3, mw 105.99 g/mol
• tris-(hydroxymethyl)aminomethane (TRIS or THAM)
  (CH2OH)3CNH2, mw 121.14 g/mol