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Moles confuse me- why are they used?

Think of moles as a "chemist's dozen". Just as 12 eggs is a dozen eggs, 6.02 × 1023 eggs is a mole of eggs. 6.02 × 1023 molecules of oxygen is a mole of oxygen.

The number of grams in a mole is different from substance to substance. If you're like most students, it's this that's confusing you. Picture it this way: a dozen elephants have a different weight than a dozen rabbits- but in each case, you have a dozen animals. Similarly, a mole of oxygen gas has a different weight than a mole of water- but in each case, you have 6.02×1023 molecules.

Why use moles? You often want to know how many molecules you have in a sample of a substance. Counting the molecules individually would be completely impractical. Even if you had a way to see the individual molecules, there are just too many, even in a tiny sample. Moles were defined to solve the problem of counting large numbers of molecules. With moles, you count the number of molecules in the sample by weighing it.

Learn to convert between moles and molecules, and moles and grams.

  • Using moles to count molecules. A mole of any substance is 6.02×1023 molecules of that substance. You can use this as a conversion factor to turn moles into molecules. For example, to figure out how many molecules there are in 0.001 moles of water,

    0.001 moles water × (6.02×1023 water molecules
    1 mole water
    ) = 6.02×1020 water molecules

    You can go the other way, too. To find how many moles are in 100 water molecules,

    100 water molecules × (1 mole water
    6.02×1023 water molecules
    ) = 1.66× 10-22 moles water

  • Using masses to count moles. A mole of any substance is the molecular weight of that substance in grams. For example, H2O has a molecular weight of 18.0 (16.0 for the oxygen and 1.0 for each hydrogen), so the mole-to-gram relation for water is

    1 mole water = 18 g water

    You can use this as a conversion factor to turn grams to moles. For example, to figure out how many moles there are in 100.0 g of water,
    100.0 g water × (1 mole water
    18.0 g water
    ) = 5.56 mole water
    and, you can go the other way. To find out how many grams there are in 6.0 moles of O2,
    6.0 moles O2 × (32.0 g O2
    1 mole O2
    ) = 192 g O2

Author: Fred Senese senese@antoine.frostburg.edu

General Chemistry Online! Moles confuse me- why are they used?

Copyright © 1997-2010 by Fred Senese
Comments & questions to fsenese@frostburg.edu
Last Revised 02/15/10.URL: http://antoine.frostburg.edu/chem/senese/101/moles/faq/why-use-moles.shtml