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  • 1. Stoichiometry: Determining Formulas Lecture 7
  • 2. Chemistry is a practical science.
  • 3. Whether you are a polymer chemist
  • 4. or a chemical engineer at NASA
  • 5. or an environmental chemist,
  • 6. Stoichiometry is what you need.
  • 7. Stoichiometry is the calculation of measurable relationships of the reactants and products in chemical reactions.
  • 8. How do chemists measure chemicals?
    • Distance is expressed in meters.
    • Mass - in kilograms.
    • Time - in seconds.
    • Luminous intensity – in candelas.
    • Temperature - in kelvins.
    • Electric current - in amperes.
    • Amount of substance is expressed in...
  • 9. To count chemical entities by weighing them chemists have devised a unit called the mole .
  • 10.
    • Lorenzo Romano Amedeo Carlo Avogadro (1776-1856), Italian scientist
  • 11. A mole is a counting unit. A pair = 2. A dozen = 12. A mole =6.022 x 10 23 .
  • 12. A mole of a specific substance represent a fixed number of chemical entities and has a specific mass.
  • 13. Atomic mass of an element expressed in amu is numerically the same as mass of one mole of the element expressed in grams.
  • 14. Numerically, atomic mass equals the mass of 1 mole:
    • 1 Fe atom has a mass of 55.85 amu
    • 1 mol of Fe atoms has a mass of 55.85 g
    • 1 S atom has a mass of 32.07 amu
    • 1 mol of S atoms has a mass of 32.07 g
    • 1 O atom has a mass of 16.00 amu
    • 1 mol of O atoms has a mass of 16.00 g
  • 15. Molecular mass of a compound expressed in amu is numerically the same as mass of one mole of the compound expressed in grams.
  • 16. Numerically, molecular (or formula) mass equals the mass of 1 mole:
    • 1 molecule of H 2 O has a mass of 18.02 amu
    • 1 mol of H 2 O (6.022 x 10 23 molecules) has a mass of 18.02 g
    • 1 formula unit of NaCl has a mass of 58.44 amu
    • 1 mol of NaCl (6.022 x 10 23 formula units) has a mass of 58.44 g
  • 17. Molar mass of a substance is the mass of its entities (atoms, molecules, or formula units).
  • 18. Use the periodic table for calculating molar mass
    • Of an element: look up the atomic mass of the element in the periodic table.
      • If the element occur as monatomic , its molar mass is that value. Just add “g/mol”.
      • If the element occur as molecular (diatomic, triatomic, etc.), its molar mass is that value multiplied by the number of its atoms. Add “g/mol”.
  • 19. Use the periodic table for calculating molar mass
    • Of a compound: look up the atomic masses of all the elements in the formula.
    • Multiply atomic mass of each element by the number of its atoms in the formula.
    • Add masses of all the elements.
    • Add “g/mol”.
  • 20. If you know the amount of a substance, you can calculate the mass of the substance.
  • 21. If you know the mass of a substance, you can calculate the amount of the substance.
  • 22. A sample problem on calculating the mass and numbers of atoms in a given number of moles of an element.
  • 23. A sample problem on calculating the moles and number of formula units in a given mass of a compound.
  • 24. A sample problem on calculating mass percents and masses of elements in a sample of a compound.
  • 25. A sample problem on determining an empirical formula from masses of elements.
  • 26. A sample problem on determining a molecular formula from elemental analysis and molar mass.
  • 27. A sample problem on determining a molecular formula from combustion analysis.
  • 28. Different compounds may have the same empirical formula.
  • 29. Different compounds may have the same molecular formula.
  • 30. THE END