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SGCh03 - C hapter Three MASS RELATIONSHIPS IN CHEMICAL...

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Chapter Three MASS RELATIONSHIPS IN CHEMICAL REACTIONS Atomic and Molecular Masses The Molar Mass of Elements and Compounds Chemical Formulas and Percent Composition Chemical Reactions and Equations Stoichiometry: Amounts of Reactants and Products Limiting Reagents and Reaction Yields ATOMIC AND MOLECULAR MASSES STUDY OBJECTIVES 1. Describe the atomic mass unit scale and the carbon-12 standard of the scale. 2. Determine the average atomic mass of an element given the masses and percent abundances of its isotopes. 3. Determine the molecular mass of a compound given its molecular formula. The Atomic Mass Unit Scale. The atomic mass unit scale works like this. The masses of individual atoms cannot be measured with a balance; but relative masses of the atoms of different elements can be measured. For instance, it is possible to determine that an atom of 4 2 He is very close to 1/3 the mass of an atom of 12 6 C. Next we assign a certain value for the mass of a carbon-12 atom. By international agreement an atom of carbon-12 has a mass of exactly 12 atomic mass units (amu). Carbon-12 is the standard of the amu scale. On this scale a helium-4 atom has a mass of 4.00 amu (1/3 of 12). Atoms heavier than carbon-12 have masses greater than 12 amu. Fluorine atoms, for instance, are 1.583 times heavier than carbon-12 atoms. Thus the mass of a fluorine atom is 19.00 amu. Other experiments showed that the lightest element is hydrogen, H atoms having about 1/12 the mass of carbon-12 atoms. In this way, the relative masses of atoms of all the elements have been established. Modern Atomic Masses. The atomic masses that appear in the present-day periodic table are determined by taking into account the fact that atoms of the elements exist as isotopes. First the mass of each isotope of an element is measured with a mass spectrometer relative to the atomic mass of carbon-12. Then the percent abundance of each isotope is measured. This information permits calculation of the average atomic mass. For example, the element lithium has two isotopes that occur in nature: 6 3 Li with 7.5 percent abundance, and 7 3 Li with 92.5 percent abundance. The atomic mass of lithium-6 is 6.01513 amu, and that of lithium-7 is 7.01601 amu. The average mass of such a mixture of Li atoms is given by: average atomic mass = (fraction of isotope X)(mass of isotope X) + (fraction of isotope Y)(mass of isotope Y) = (0.075)(6.01513 amu) + (0.925)(7.0161 amu) = 0.45 amu + 6.49 amu = 6.94 amu
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38 / Mass Relationships in Chemical Reactions Note that neither 6 3 Li nor 7 3 Li has an atomic mass of 6.94 amu. This is the average value for the mixture of the two Li isotopes. Molecular Masses. Compounds result when atoms of different elements bond to each other. There are two kinds of compounds: molecular and ionic. Molecules are tiny particles that are formed when atoms combine and are held together by chemical forces. A molecule of a compound is formed by the combination of atoms of different elements. The molecular mass is the sum of the atomic masses of the atoms in the molecular formula. For example, the molecular masses of two nitrogen oxides NO
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