Hello, Esmi,
In science, a mole is a unit of count. 1 mole mean you have 6.02x1023 items of something. A farmer might say a dozen eggs. A chemist might say 1 mole of molecules. Molecules are so small they need a separate system of nomenclature. So the mole was invented to make it easier to say "I have 600,200,000,000,000,000,000,000 salt molecules."
Just as with a dozen, we can write it with either term on top. 12 eggs/dozen is equivalent to 1 dozen/12 eggs. Both the top and bottom represent the same amount, so the expression is equivalent to "1." which we can multiply agains anything, and cancelling or adding units in the process.
So we can say either:
- (6.02x1023 molecules/1 mole), or
- 1 mole/6.02x1023molecules)
It's hard to find the English term for such larger numbers, and they become both meaningless, and difficult to communicate correctly. Instead, they say "I have one mole of salt." Why the odd number? Well, it is correlated to the molar mass of elements and compounds. The molar mass of salt is the sum of the atomic masses of Na and Cl, for a total of 58.5 amu. but expressed not as amu, but as grams/mole. It takes 58.5 grams of NaCl to have 6.02x1023 molecules of the compound. So it is a really useful conversion factor. It allows scientists to use a mass scale to measure out numbers of atoms, instead of a set of invisible tweezers to count them out one by one.
In the question you pose, we want the number of moles contained in 18.06x1023 atoms. Use the conversion factor we just discussed: You can invert a conversion factor (one side is equal to the other, so it makes no difference which is on top, but it does change the unit.)
(18.06x1023 atoms Na)*((1 mole Na)/(6.2x1023 atoms Na))
= (18.06/6.02) mole
= 3.0 moles Na
Bob
