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Unformatted text preview: 1 Composition Stoichiometry Principles of General Chemistry, 2 nd ed. By M. Silberberg Chemistry, 8th ed. by W. Whitten, R. Davis, R., M. L. Peck, and G. Stanley. 2 Lecture • The Mole • Atomic Weights • Formula Weights, Molecular Weights and Moles • Percent Composition • Derivation of Formulas from Elemental Composition • Determination of Molecular Formulas • Some Other Interpretations of Chemical Formulas • Purity of Samples The Atomic Weights • If we define the mass of 12 C as exactly 12 atomic mass units (amu), then it is possible to establish a relative weight scale for atoms. – 1 amu = (1/12) mass of 12 C by definition 3 • Atomic Weight - weighted average of the masses of the constituent isotopes of an element. • Example. Naturally occurring Cu consists of 2 isotopes. It is 69.1% 63 Cu with a mass of 62.9 amu, and 30.9% 65 Cu, which has a mass of 64.9 amu. Calculate the atomic weight of Cu to one decimal place. 4 The Atomic Weights • Example. The atomic weight of boron is 10.811 amu. The masses of the two naturally occurring isotopes 5 10 B and 5 11 B, are 10.013 and 11.009 amu, respectively. Calculate the fraction and percentage of each isotope. 5 The Atomic Weights 6 The Mole • A number of atoms, ions, or molecules that is large enough to see and handle. • A mole = number of things – Just like a dozen = 12 things – One mole = 6.022 x 10 23 things • Avogadro’s number = 6.022 x 10 23 – Symbol for Avogadro’s number is N. One mole (1 mol) contains 6.022 x 10 23 entities (to four significant figures) 7 Interconverting Moles, Mass, and Number of Chemical Entities Mass (g) = no. of moles x no. of grams 1 mol No. of moles = mass (g) x no. of grams 1 mol No. of entities = no. of moles x 6.022 x 10 23 entities 1 mol No. of moles = no. of entities x 6.022 x 10 23 entities 1 mol g No. of moles = mass (g) x no. of grams 1 mol mol 8 • Example: Calculate the mass of a single Mg atom in grams to 3 significant figures....
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