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.
|0.001 moles water ×||(||6.02×1023 water molecules|
1 mole water
|)||= 6.02×1020 water molecules|
|100 water molecules ×||(||1 mole water|
6.02×1023 water molecules
|)||= 1.66× 10-22 moles water|
1 mole water = 18 g waterYou 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|
|6.0 moles O2 ×||(||32.0 g O2|
1 mole O2
|)||= 192 g O2|
Author: Fred Senese firstname.lastname@example.org
Copyright © 1997-2010 by Fred Senese
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Last Revised 08/17/15.URL: http://antoine.frostburg.edu/chem/senese/101/moles/faq/print-why-use-moles.shtml