1027 Thomas Chapter 6

1027 Thomas Chapter 6 - Chapter 4 Formula Mass and the Mole...

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Chapter 4 Formula Mass and the Mole
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Formula Mass The mass of an atom is called its atomic mass. The molecular mass is the sum of the masses of the atoms represented in the molecular formula Molecular mass is used specifically for molecules Ionic compounds do not exist as molecules Formula mass is the sum of all the masses of the atoms or ions present in the formula unit
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Calculating mass NaCl (formula unit) Formula mass Na = 1 x 22.98977 = 22.98977 amu Cl = 1 x 35.453 = 35.453 amu 58.442 amu ~58 amu C 6 H 12 O 6 (molecule) Molecular mass C = 6 x 12.011 = 72.066 amu H = 12 x 1.00794 = 12.09528 amu O = 6 x 15.9994 = 95.9964 amu 180.157 amu ~ 180 amu
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Determine the Formula mass of Ammonium sulfate Ammonium sulfate (NH 4 ) 2 SO 4 (Remember, if you can not write correct formulas, you can not calculate the correct mass) N: 2 x 14 = 28 H: 8 x 1 = 8 S: 1 x 32 = 32 O: 4 x 16 = 64 Total = 132 u
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The Mole You purchase a dozen of eggs, or a case of soda from the market. You may order a ream of paper for the office Dozen (12) , case (24) , or a ream of paper (500) or pens by the gross (144 ); a particular number comes to mind In chemistry, particles such as atoms, ions, molecules are counted by the mole (mol) One mol contains a very large number of particles 1 mole contains Avogadro’s number of particles 1 mol = 6.02 x 10 23 particles
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The Mole Atomic mass provides a means to count atoms by measuring the mass of a sample The periodic table gives atomic masses of the elements The mass of an atom is called its atomic mass
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The molecular mass allows counting of molecules by mass 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 Strictly speaking, ionic compounds do not have a “molecular mass” because they don’t contain molecules
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The mass of the formula unit is called the formula mass 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
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One mole of a substance has a mass in grams numerically equal to its formula mass The mass of one mole of a substance is also called its molar mass One mole of any substance contains the same number of formula units This number is called Avogadro’s number or constant 1 mol formula units = 6.02 x 1023 formula units
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Counting formula units by moles is no different than counting eggs by the dozen (12 eggs) or pens by the gross (144 pens) Avogadro’s number is huge because atoms and molecules are so small: a huge number of them are needed to make a lab-sized sample Avogadro’s number links moles and atoms, or moles and molecules and provides an easy way to link mass and atoms or molecules
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Using water (molar mass 18.015) as an
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1027 Thomas Chapter 6 - Chapter 4 Formula Mass and the Mole...

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