Unit 1 - Chapter 3 - Zumbahl Chemistry 6th Edition Chapter...

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Zumbahl: Chemistry 6 th Edition Chapter 3: Stoichiometry Overview Chemical stoichiometry – the calculation of the quantities of material consumed and produced in chemical reactions. 3.1 Atomic Mass Atomic mass or atomic weight – the weighted average of mass of the atoms in a naturally occurring element. Mass spectrometer – an instrument used to determine the relative masses of atoms by the deflection of their ions on a magnetic field. ∙Atoms or molecules are passed into a beam of high-speed electrons, which knock electrons off the atoms or molecules being analyzed and change them into positive ions. An applied electric field then accelerates these ions into a magnetic field. The amount of path deflection for each ion depends on its mass - the most massive ions are deflected the smallest amount – which causes the ions to separates. ∙A comparison of the positions where the ions hit the detector plate gives very accurate values of their relative masses. ∙Most elements occur in nature as mixtures of isotopes; thus atomic masses are usually average values. 3.2 The Mole Mole , or mol – the number equal to the number of carbon atoms in exactly 12 grams of pure 12 C; Avogadro’s number. ∙Also defined such that a sample of a natural element with a mass equal to the element’s atomic mass expressed in grams contains 1 mole of atoms. Avogadro’s number – the number of atoms in exactly 12 grams of pure 12 C; equals to 6.022 x 10 23 . ∙This means that 12 grams of 12 C contains 6.022 x 10 23 atoms. ∙This definition also fixes the relationship between the atomic mass unit and the gram. ∙(6.022 x 10 23 atoms )(12 amu ) = 12 g atom ∙6.022 x 10 23 amu = 1 g (exact number) ∙This relationship can be used to derive the unit factor needed to convert between atomic mass units and grams. ∙See Sample Exercise 3.2 on page 87. ∙One mole of something consists of 6.022 x 10 23 units of that substance. ∙For example: a dozen eggs is 12 eggs, a mole of eggs is 6.022 x 10 23 eggs. ∙The mass of 1 mole of an element is equal to its atomic mass in grams. 3.3 Molar Mass ∙A chemical compound is a collection of atoms. ∙For example: Methane consists of one carbon and four hydrogen. ∙To calculate the mass of 1 mole of a compound, add the masses of elements present. ∙For example: Methane Mass of 1 mole C = 12.01 g Mass of 1 mole H = 4 x 1.008 g Mass of 1 mole of CH 4 = 16.04 g
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Zumbahl: Chemistry 6 th Edition Molar mass – the mass in grams of one mole of molecules or formula units of a substance; also called molecular weight. ∙A substance’s molar mass is the mass in grams of 1 mole of the substance. 3.4 Percent Composition of Compounds ∙There are two common ways of describing the composition of a compound: 1. In terms of the numbers of its constituent atoms.
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