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# Lecture07221

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a supplemental resource for students

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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.