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chp10_slides - The Mole Weve seen the atomic mass unit(amu...

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The Mole We’ve seen the atomic mass unit ( amu or μ ) de- fined as being equal to exactly 1 12 the mass of an atom of Carbon-12. The atomic masses of the other elements are also in amu and are therefore being expressed relative to the mass of the Carbon-12 atom. A mole is defined as the number of Carbon-12 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 element’s 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 Avogadro’s 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
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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
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A sample problem from the last slide What is the mass of 1 . 23 * 10 11 sulfur atoms in grams? First, let’s 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
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A sample problem (cont’d) Next, let’s 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
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The Mole (cont’d) If I have a mole of some type of molecular com- pound , then for each atom’s occurrence in the molecular formula of the molecule, I have a mole of that atom. 1. Example: In H 2 O : I have 2 occurrences of H and 1 of O So the mass of a mole of H 2 O atoms equals : the mass of 2 moles of H atoms plus the mass of 1 mole of O atoms which is 18.0g. The molar mass of H 2 O is 18.0 g . 2. Example: In C 6 H 12 O 6 : I have 6 C ’s, 12 H ’s, and 6 O ’s So the mass of a mole of C 6 H 12 O 6 molecules equals: the mass of 6 moles of C atoms plus the mass of 12 moles of H atoms plus the mass of 6 moles of O atoms = 180. g.
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