When working chemistry problems, it is useful to know how many atoms or molecules you have in your sample. This is because atoms weigh different amounts: one water and two hydrogen atoms react to form one water molecule, but two grams of hydrogen and one gram of oxygen do not form three grams of water.

The quantity that chemists use when disscussing number of atoms is the mole. A mole is a quantity of something just like one, two or a dozen: you can have a single atom, two atoms, a dozen atoms, or a mole of atoms. The number of things in a mole is Avagadro's number

N_a = 6.022*10²³

Example: How many water molecules are in 0.500 moles?

Solution: There are 6.022*10²³ molecules in a mole. Since we have 0.500 moles,

0.500 moles * 6.022*10²³ atoms/mole = 3.011*10²³ molecules

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