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Stoichiometric Calculations: The Workhorse of the Analyst

Stoichiometric Calculations: The Workhorse of the Analyst. Review of Fundamentals. Atomic, Molecular, and Formula Weights Moles: 1mole = 6.022 x 10 23 (atoms, molecules or formula units). How Do We Express Concentrations of Solutions ? . Molarity (M)= moles/liter or mmoles / mL

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Stoichiometric Calculations: The Workhorse of the Analyst

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  1. 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/1000 g solvent

  4. In normality calculations, the number of equivalents is the number of moles times the number of reacting units per molecule or atom.

  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. • MstockxmLstock = Mdilutedx 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 106ppm • 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 106ppm • ppb(wt/vol) = (wt analyte/volsample,mL)x 109 ppb • Liquid Analyte • %(vol/vol) = (volanalyte/vol sample mL)x 102 % • pt(vol/vol) = (volanalyte/vol sample mL)x 103 ppt • ppm(vol/vol) = (volanalyte/vol sample mL)x 106ppm • ppb(vol/vol) = (volanalyte/volsample,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.

  11. The equivalents (based on charge) of cations and anions are equal

  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. The coefficients in the balanced equation give the ratio of moles of reactants and products

  14. From the mass of Substance A you can use the ratio of the coefficients of A and B to calculate the mass of Substance B formed (if it’s a product) or used (if it’s a reactant)

  15. C6H12O6 + 6 O2 6 CO2 + 6 H2O Starting with 1.00 g of C6H12O6… we calculate the moles of C6H12O6… use the coefficients to find the moles of H2O… and then turn the moles of water to grams

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