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Lecture4sf-1

# Lecture4sf-1 - The Mole Concept Consider the following...

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The Mole Concept Consider the following approximate atomic masses of some elements. These numbers represent the relative mass of each element. The units are atomic mass units (abbreviated amu, and given the symbol u), a very small unit of mass. H 1.0 u He 4.0 u C 12.0 u O 16.0 u Cu 63.5 u Pb 207 u U 238 u This scale tells us relative masses, with carbon-12 being defined as exactly 12 u. A helium atom has a mass 4 times that of hydrogen. A carbon atom has a mass 3 times that of helium (ratio of 12/4). A uranium atom is almost 20 times heavier than a carbon atom (ratio of 238/12). Now suppose we have a number of atoms (called an amount ) that has a mass equal to the atomic masses in grams. Let’s call this amount N atoms. N atoms of hydrogen weigh 1.0 grams What would be the mass of N atoms of helium? Since each helium atom weighs 4 times that of each hydrogen atom, the answer is 4.0 grams. What would be the mass of N atoms of oxygen? Since each oxygen atom weighs 16 times that of each hydrogen atom, the answer is 16.0 grams. This reasoning should be clear: The same N atoms (the same amount) of each element would have a mass equal to its atomic mass in grams.

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Mass N atoms of hydrogen 1.0 g N atoms of helium 4.0 g N atoms of carbon 12.0 g N atoms of oxygen 16.0 g N atoms of copper 63.5 g N atoms of lead 207 g N atoms of uranium 238 g This amount, the number of atoms that has a mass equal to the atomic mass in grams, is called a mole . A mole of an element always has the same amount of atoms, so far designated by N. But a mole does not have to refer only to atoms. We can have compounds in which a molecule (a group of atoms) is the smallest unit. On the atomic mass scale: Mass 1 molecule H 2 O 18.0 u 1 molecule CO 2 44.0 u 1 molecule C 6 H 12 O 6 180 u Here we find the mass of each molecule by just adding up the atomic masses in the formula. By the same reasoning illustrated above: Mass N atoms of carbon 12.0 g N molecules of H 2 O 18.0 g N molecules of CO 2 44.0 g N molecules of C 6 H 12 O 6 180 g N represents the number of elementary units in a mole. For elements, the elementary unit is an atom; for compounds, the elementary unit is a molecule.
Definition of A Mole The mole is the SI unit of amount. An atom of carbon-12 has a mass of exactly 12 u. (this defines the amu scale)

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Lecture4sf-1 - The Mole Concept Consider the following...

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