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Stoichiometric Calculations

Stoichiometric Calculations. The workhouse of the analyst. Review of the Fundamentals. Molarity (M): moles/Liter Molality (m): moles/Kg. Normality (N). The normality of a solution is simply a multiple of the molarity of the solution.

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Stoichiometric Calculations

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  1. Stoichiometric Calculations The workhouse of the analyst

  2. Review of the Fundamentals • Molarity (M): moles/Liter • Molality (m): moles/Kg

  3. 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.

  4. 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.

  5. 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.

  6. 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.

  7. 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.

  8. Standardizing Acids • Sodium Carbonate • 2NaHCO3(s)→Na2CO3(s)+H2O(g)+CO2(g) Heat purified sodium hydrogen carbonate between 270 and 300oC. • The point at pH=3.8 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.

  9. Preparing Standard Base Solutions • Sodium hydroxide is the most common base for preparing standard solutions. • The effect of CO2:CO2+2OH-→CO3-2+H2O • If the acidic solution: CO3-2+H3O+→H2CO3+2H2O • In the basic solution: CO3-2+H3O+→HCO3-+H2O (carbonate error)

  10. 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.

  11. 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.

  12. 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 Some primary standards for titration of bases

  13. 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

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