Section Five

1. Section Five Stoichiometric Calculations: The Workhorse of the Analyst

2. Review of Fundamentals • Atomic, Molecular, and Formula Weights • Moles: 1mole = 6.022 x 1023 (atoms, molecules or formula units)

3. How Do We Express Concentrations of Solutions? • Molarity (M)= moles/liter or mmoles/mL • Normality(N) = equivalence/liter or meq/mL • Formality(F)= is identical to molarity • Molality(m) = moles/1000g solvent

4. In normality calculations, the number of equivalents is the number of moles times the number of reacting units per molecule or atom. ©Gary Christian, Analytical Chemistry, 6th Ed. (Wiley)

5. Density CalculationsHow do we convert to Molarity • Density = mass solute /unit volume • Specific Gravity = Dsolute/DH20 • DH2O = 1.00000 g/mL @ 4oC • DH2O = 0.99821 g/mL @ 20oC

6. Analytical and Equilibrium Concentrations • They are not the same! • Analytical Molarity, Cx = sum of all species of the substance in solution • Equilibrium Molarity, [X] = concentration of a given dissolved form of the substance

7. DilutionsPreparing the Right Concentration • The millimoles taken for dilution will be the same as the millimoles in the diluted solution. • Mstockx mLstock = Mdiluted x mLdiluted

8. Expression of Analytical ResultsSo Many Ways • Solid Samples: • %(wt/wt) = (wt analyte/wt sample)x 102 % • pt(wt/wt) = (wt analyte/wt sample)x 103 ppt • ppm(wt/wt) = (wt analyte/wt sample)x 106 ppm • ppb(wt/wt) = (wt analyte/wt sample)x 109 ppb

9. Expression of Analytical ResultsSo Many Ways • Liquid Samples • %(wt/vol) = (wt analyte/vol sample mL)x 102 % • pt(wt/vol) = (wt analyte/vol sample mL)x 103 ppt • ppm(wt/vol) = (wt analyte/vol sample mL)x 106 ppm • ppb(wt/vol) = (wt analyte/vol sample,mL)x 109 ppb • Liquid Analyte • %(vol/vol) = (vol analyte/vol sample mL)x 102 % • pt(vol/vol) = (vol analyte/vol sample mL)x 103 ppt • ppm(vol/vol) = (vol analyte/vol sample mL)x 106 ppm • ppb(vol/vol) = (vol analyte/vol sample,mL)x 109 ppb

10. The units ppm or ppb are used to express trace concentrations. These are weigh or volume based, rather than mole based. ©Gary Christian, Analytical Chemistry, 6th Ed. (Wiley)

11. The equivalents (based on charge) of cations and anions are equal. ©Gary Christian, Analytical Chemistry, 6th Ed. (Wiley)

12. Reporting Concentrations as Different Chemical Species • We may express results in any form of the analyte. • Example: • Water Hardness due to calcium ion is expressed as ppm CaCO3. • Chloride present in AgCl derived from a salt mixture of NaCl and KCl.

13. Volumetric Analysis - Principles • “Titrimetry” – determination of analyte by reaction with measured amount of standard reagent • “Standard Solution” (titrant) – reagent of known concentration • “Titration” – slow addition of titrant to analyte solution from a volumetric vessel (buret) • “Equivalence Point” – reached when amount of added titrant is chemically equivalent to amount of analyte present in the sample. • “End Point” – the occurrence of an observable physical change indicating that the equivalence point is reached. Might differ from Eq.Pt.!

14. Volumetric Analysis - PrinciplesHow Do We Make Stoichiometric Calculations? • Titration- What are the requirements? • Reaction must be stoichiometric • Reaction should be rapid • No side reactions • Marked change in some property of the solution when reaction is complete • Equivalence point • Reaction should be quantitative

15. Volumetric Analysis - Principles Standard Solutions • “Primary Standard “ – • highly purified compound used as a reference material in titrimetry • Properties: • High purity • Stable in air • Independent of relative humidity • Readily available • Reasonable solubility • Large formula weight

16. Volumetric Analysis - Principles Standard Solutions • “Secondary Standard” – do not meet requirements for a primary standard but are available with sufficient purity and properties to be generally acceptable • Desirable properties of a Standard Solution: • Prepared from primary standard • Stable • Reacts rapidily and completely with analyte • Reacts selectively with analyte

17. Primary Potassium Acid Phthalate KHC8H4O4 (FW 204.23) Benzoic Acid C6H5COOH (FW 122.12) Na2CO3 , KH(IO3)2 Arsenious Oxide (As2O3) Sodium Oxalate (Na2C2O4) KI , K2Cr2O7 , Fe(pure) Secondary NaOH , KOH , Ba(OH)2 HCl , HNO3 , HClO4 Sulfamic Acid (HSO3NH2) KMnO4 , Na2S2O3 Ce(HSO4)4 (FW 632.6) Volumetric Analysis - PrinciplesExamples of Standard Materials

18. Volumetric Analysis-Principles • Standardization – involves establishing the concentration of a “standard solution” • Direct method: • dissolve caarefully weighed quantity of primary standard; dilute to known volume • Indirect methods: • Titrate weighed quantity of primary standard • Titrate weighed quantity of secondary standard • Titrate measured volume of other standard solution

20. Volumetric Analysis - Principles • Acid – Base • HA + B  BH+ + A- • Precipitation • Mn+ + X-  MXn • Oxidation-Reduction • Red1 + Ox2Ox1 + Red2 • Complexation • Mn+ + Q  M(Q)n+ use standard Q solution

21. Volumetric Analysis - Principles • Dilutions • Vc x Cc = Vd x Cd • Stoichiometric Ratios (S.R.) Mole Ratio: • What are stoichiometric ratios (mole ratios)? • 2 HCl + Ba(OH)2  2 HOH + Ba2+ + 2 Cl- • 2 MnO4- + 5 C2O42- + 16 H+  2Mn2+ + 10 CO2 + 8 HOH