Chapter 3

Chapter 3 - Chapter 3 Page 1 Chemical soichiometry The...

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Page # 1 Chemical soichiometry - The calculation of the quantities of material consumed and produced in chemical reactions. 3.1 Atomic Masses Ÿ The modern system of atomic masses, instituted in 1961, is based on carbon-12 as the standard. In this system carbon-12 is assigned a mass of exactly 12 atomic mass units (amu), and the masses of all other atoms are given relative to this standard. Ÿ The most accurate method currently available for comparing the masses of atoms involves the use of the mass spectrometer. Ÿ Because natural carbon is a mixture of isotopes, the atomic mass we use for carbon is an average value based on its isotopic composition Ÿ This average mass is often called the atomic weight of carbon 3.2 The Mole Ÿ The mole (abbreviated mol) as the number equal to the number of carbon atoms in exactly 12 grams of pure carbon 12 Ÿ Avogadro’s number is 6.022 * 10^23 (6.022*10^23 amu = 1 gram) Ÿ The mole is 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. Ÿ Calculate from GRAMS to MOLES, MOLES to ATOMS 3.3 Molar Mass Ÿ A chemical compound is, ultimately, a collection of atoms. Ÿ The mass of 1 mole of methane can be found by summing the masses of carbon and hydrogen present Ÿ The terms molar mass and molecular weight mean exactly the same thing: the mass in grams of 1 mole of a compund. Ÿ The molar mass of a known substance is obtained by summing the masses of the component atoms Ÿ In some texts the term formula weight is used for ionic compounds instead of the terms molar mass and molecular weight. 3.4 Percent Composition of Compounds Ÿ It is often useful to know a compound’s composition in terms of the masses of its elements. We can obtain this information from the formula of the compound by comparing the mass of each element present in 1 mole of the compound with the total mass of 1 mole of the compound. 3.5 Determining the Formula of a Compound

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Chapter 3 - Chapter 3 Page 1 Chemical soichiometry The...

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