Chapter%204%20Outline - Chemistry 101 Vandan Desai Chapter...

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Chemistry 101 Vandan Desai P a g e | 1 Chapter 4: The Mole – Connecting the Macroscopic and Molecular Worlds (Lecture Outline) -------------------------------------------------------------------------------------------------------------------------- The essential link between the small-scale world of atoms and molecules and large-scale mass measurements is the mole . Part of chemistry that deals with these quantitative relationships is called stoichiometry . I. Use large-scale measurements to count tiny objects A. Count atoms in a mass of element using atomic masses i. Atomic mass provides a means to count atoms by measuring mass of a sample ii. The periodic table gives atomic masses of the elements iii. The mass of an atom is called its atomic mass iv. 1 amu (u) = 1.661 x 10 -27 kg v. When using atomic masses, retain a sufficient number of significant figures so the atomic mass data contributes only slightly to the uncertainty of the result B. Mass measurements can be used to count molecules using molecular masses i. Molecular mass allows counting of molecules by mass ii. The molecular mass is the sum of atomic masses of the atoms in the compounds formula For example the molar mass of water, H 2 O, is twice the mass of hydrogen (1.008) plus the mass of oxygen (15.999) = 18.015 iii. Strictly speaking, ionic compounds don’t have a “molecular mass” b/c they don’t contain molecules iv. The mass of the formula unit is called the formula mass v. Formula masses are calculated the same way as molecular masses For example the formula mass of calcium oxide, CaO, is the mass of calcium (40.08) plus the mass of oxygen (15.999) = 56.08 II. The mole conveniently links mass to number of atoms or molecules A. One mole of a substance contains the same number of formula units as the number of atoms in exactly 12 g of carbon-12 B. One mole of a substance has a mass in grams numerically equal to its formula mass C.
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