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Unformatted text preview: The Mole Weve seen the atomic mass unit ( amu or ) de fined as being equal to exactly 1 12 the mass of an atom of Carbon12. The atomic masses of the other elements are also in amu and are therefore being expressed relative to the mass of the Carbon12 atom. A mole is defined as the number of Carbon12 atoms that has a mass equal to 12 grams. 1. The mole is a convenient number . As it is convenient to speak of a dozen eggs or a gross of nails, so it is convenient to speak of a mole of atoms or a mole of molecules or a mole of ions . 2. A mole of atoms of a particular element has a mass ( in grams ) equal to the value of the elements atomic mass. We call this the molar mass of the element and denote it by M . 3. The number represented by a mole is called Avogadros Number and is: N a = 6 . 02 * 10 23 (a) A mole of H atoms has a mass of 1.00 g. (6 . 02 * 10 23 H atoms has a mass of 1.00 g) (b) A mole of Cr atoms has a mass of 52.0 g. (6 . 02 * 10 23 Cr atoms has a mass of 52.0 g) 1 Please Answer these questions 1. What is the mass of a mole of Li atoms? 2. What is the mass of half a mole of Fe atoms? 3. What is the mass of 3 . 01 * 10 23 Fe atoms? 4. What is the mass of 1 . 23 * 10 22 sulfur atoms? 5. How many moles are in 8 grams of He ? 6. What is the mass of 6 moles of C atoms? 7. What is the mass of 12 moles of H atoms? 8. What is the mass of 6 moles of O atoms? 2 A sample problem from the last slide What is the mass of 1 . 23 * 10 11 sulfur atoms in grams? First, lets find the number of moles in this number of atoms. Let n a = the number of sulfur atoms (1 . 23 * 10 22 atoms) (given info). Let n m = the number of moles of sulfur atoms (what we wish to find) (units are moles or mol) So: n a atoms is to n m moles as 6 . 02 * 10 23 atoms is to 1 mole, or: n a atoms n m moles = 6 . 02 * 10 23 atoms 1 mole So solving for n m : n m mol = 1 mol 6 . 02 * 10 23 atoms * n a atoms = 1 mol 6 . 02 * 10 23 atoms * 1 . 23 * 10 22 atoms = . 0204 mol 3 A sample problem (contd) Next, lets find the mass of .0204 moles of Sulfur. Let m = the mass of the n m (= .0204) moles of Sulfur, Then: m grams is to n m moles as 32 . 1 grams is to 1 mole, or: m grams n m moles = 32 . 1 grams 1 mole This is because the atomic mass of Sulfur is 32.1, which we can read as 32.1 grams per mole. Solving for m : m grams = 32 . 1 grams 1 mol * n m mol = 32 . 1 grams 1 mole * . 0204 moles = . 655 g Notice my careful accounting for units throughout the calculation; it helps me keep my place. 4 The Mole (contd) If I have a mole of some type of molecular com pound , then for each atoms occurrence in the molecular formula of the molecule, I have a mole of that atom....
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 Spring '08
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 Physics, Mass

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