Chapter 4 Power Point Notes

Chapter 4 Power Point Notes - Chapter 4: The Mole Atomic...

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1 Chapter 4: The Mole • Atomic mass provides a means to count atoms by measuring the mass of a sample • The periodic table on the inside cover of the text gives atomic masses of the elements • The mass of an atom is called its atomic mass • 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 • 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 • 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 • One mole of a substance contains the same number of formula units as the number of atoms in exactly 12 g of carbon-12 • 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 10 23 formula units • 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 • Using water (molar mass 18.015) as an example: 1 mole H 2 O Ù 6.022 x 10 23 molecules H 2 O 1 mole H 2 O Ù 18.015 g H 2 O 18.015 g H 2 O Ù 6.022 x 10 23 molecules H 2 O • Within chemical compounds, moles of atoms always combine in the same ratio as the individual atoms themselves so: 1 mole H 2 O Ù 2 mole H 1 mole H 2 O Ù 1 mole O
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2 Stoichiometry is the study of the mass relationships in chemical compounds and
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Chapter 4 Power Point Notes - Chapter 4: The Mole Atomic...

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