Moles confuse me- why are they used?

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