Brady Chapter04 - 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

# Brady Chapter04 - Chapter 4 The Mole Atomic mass provides a...

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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 6.022 x 10 23 molecules H 2 O 1 mole H 2 O 18.015 g H 2 O 18.015 g H 2 O 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 2 mole H
Stoichiometry is the study of the mass relationships in chemical compounds and

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