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Law of Constant Composition

Law of Constant Composition. All pure substances have constant composition. All samples of a pure substance contain the same elements in the same percentages (ratios). Mixtures have variable composition.

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Law of Constant Composition

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  1. Law of Constant Composition • All pure substances have constant composition. • All samples of a pure substance contain the same elements in the same percentages (ratios). • Mixtures have variable composition.

  2. Practice—Show that Hematite Has Constant Composition if a 10.0 g Sample Has 7.2 g Fe and the Rest Is Oxygen; and a Second Sample Has 18.1 g Fe and 6.91 g O.

  3. Formulas Describe Compounds Water = H2O \ two atoms of hydrogen and 1 atom of oxygen Table sugar = C12H22O11\12 atoms of C, 22 atoms of H and 11 atoms O

  4. Classifying Materials • Atomic elements = Elements whose particles are single atoms. • Molecular elements = Elements whose particles are multi-atom molecules. • Molecular compounds = Compounds whose particles are molecules made of only nonmetals. • Ionic compounds = Compounds whose particles are cations and anions.

  5. Molecular Elements • Certain elements occur as diatomic molecules. • 7 diatomic elements—

  6. Molecular Compounds • Two or more nonmetals. • Smallest unit is a molecule.

  7. Ionic Compounds • Metals + nonmetals. • No individual molecule units, instead have a 3-dimensional array of cations and anions made of formula units.

  8. Molecular View of Elements and Compounds

  9. Classify Each of the Following as Either an Atomic Element, Molecular Element, Molecular Compound, or Ionic Compound. • Aluminum, Al. • Aluminum chloride, AlCl3. • Chlorine, Cl2. • Acetone, C3H6O. • Carbon monoxide, CO. • Cobalt, Co.

  10. Ionic Nomenclature Ionic Formulas • Write each ion, cation first. Don’t show charges in the final formula. • Overall charge must equal zero. • If charges cancel, just write symbols. • If not, use subscripts to balance charges. • Use parentheses to show more than one polyatomic ion. • Stock System - Roman numerals indicate the ion’s charge.

  11. Ionic Nomenclature Ionic Names • Write the names of both ions, cation first. • Change ending of monatomic ions to -ide. • Polyatomic ions have special names. • Stock System - Use Romannumerals to show the ion’s charge if more than one is possible. Overall charge must equal zero.

  12. Ionic Nomenclature • Consider the following: • Does it contain a polyatomic ion? • -ide, 2 elements  no • -ate, -ite, 3+ elements  yes • Does it contain a Roman numeral? • Most transition metals, Sn and Pb. • No prefixes!

  13. Ionic Nomenclature Common Ion Charges 1+ 0 2+ 3+ NA 3- 2- 1-

  14. Ionic Nomenclature • potassium chloride • magnesium nitrate • copper(II) chloride

  15. Ionic Nomenclature • NaBr • Na2CO3 • FeCl3

  16. Molecular Nomenclature • Prefix System (binary compounds) 1. Less e-neg atom comes first. 2. Add prefixes to indicate # of atoms. Omit mono- prefix on first element. 3. Change the ending of the second element to -ide.

  17. PREFIX mono- di- tri- tetra- penta- hexa- hepta- octa- nona- deca- NUMBER 1 2 3 4 5 6 7 8 9 10 Molecular Nomenclature

  18. Molecular Nomenclature • CCl4 • N2O • SF6

  19. Molecular Nomenclature • arsenic trichloride • dinitrogen pentoxide • tetraphosphorus decoxide

  20. Molecular Nomenclature • The Seven Diatomic Elements Br2 I2 N2 Cl2 H2 O2 F2 H N O F Cl Br I

  21. Definition • Acids • Compounds that form H+ in water. • Formulas usually begin with ‘H’. • Examples: • HCl – hydrochloric acid • HNO3 – nitric acid • H2SO4 – sulfuric acid

  22. Acid Nomenclature

  23. Acid Nomenclature

  24. Acid Nomenclature • HBr • H2CO3 • H2SO3 hydrobromic acid • 2 elements, -ide • 3 elements, -ate  carbonic acid  sulfurous acid • 3 elements, -ite

  25. Acid Nomenclature • hydrofluoric acid • sulfuric acid • nitrous acid • 2 elements  H+ F-  HF • 3 elements, -ic  H+ SO42-  H2SO4  H+ NO2-  HNO2 • 3 elements, -ous

  26. Formula Mass • The mass of an individual molecule or formula unit. • Also known as molecular mass or molecular weight. • Sum of the masses of the atoms in a single molecule or formula unit. • Whole = Sum of the parts. Mass of 1 molecule of H2O = 2(1.01 amu H) + 16.00 amu O = 18.02 amu.

  27. Practice—Calculate the Formula Mass of Al2(SO4)3.

  28. Practice—Calculate the Formula Mass of Al2(SO4)3, Continued.

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