2. Atoms, Molecules and Moles   Previous PageNext Page
     Molecules, Molecular Weight, and Moles

Chemists talk about reactions between molecules, yet except for certain extraordinary experimental conditions, no one can see a molecule. There is no easy way to count out equal numbers of various kinds of molecules in preparation for a chemical reaction.

There is a simple way, however, to weigh different amounts of various molecules and to be sure that the resulting amounts each contain the same number of molecules.

Since the molecular weights of hydrogen gas, methane, and water are 2.016 amu, 16.043 amu, and 18.015 amu, respectively, we can be sure that 2.016 tons of hydrogen gas, 16.043 tons of methane, and 18.015 tons of water each contain the same number of molecules, although we may have no idea what that number is.

By the same principle, if we know that walnuts weigh twice as much as peanuts, we can be sure that two pounds of walnuts and one pound of peanuts contain the same number of nuts, without counting them or knowing exactly how many there are. If our only goal is to pair off walnuts with peanuts, or to pair off molecules in chemical reactions, then this limited information is good enough.

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