Question

What is meant by the term 'mole' (and its unit, 'mol') in chemistry? (Hint: not a small burrowing mammal)

Answer 1

A `"mol"` is essentially a large quantity, equal to about `6.022xx10^23` (Avogadro's number).

The number is chosen, though, so that a large quantity of quantum-sized objects (i.e. atoms, molecules, etc) can be described in more manageable, less awkward values.

For instance, instead of saying we have `6.022xx10^23` atoms, which is a bit awkward, it is more convenient to say:

`6.022xx10^23 cancel"atoms" xx ("1 mol")/(6.022xx10^23 cancel"atoms")`

`=` `color(blue)("1 mol of atoms")`

It's just a new unit to express a large quantity of things.

`"1 mol"` of pictures is the same quantity as `"1 mol"` of thumbdrives is the same quantity as `"1 mol"` of paper towels. They aren't the same mass, but there are the same number of them.

We have defined our units so that, for example, the atomic mass of `""_6^12"C"`, the carbon-12 isotope, is `"12 amu"` (atomic mass units). That is the mass of one atom of carbon-12. But if we express the mass as `"12 g"` instead, that is the mass of `"1 mol"` of carbon-12 atoms.

The `"mol"` concept is handy because `"1.0079 g"` of `"H"` is `"1 mol"` and `"15.999 g"` of `"O"` is `"1 mol"`, etc.

Suppose we have `"25 g"` of `"N"_2` and want to know how many `"mol"`s that is.

The molar mass of `"N"` atom is `"14.007 g/mol"`, so the molar mass of `"N"_2` is `"28.014 g/mol"`.

`25 cancel("g N"_2) xx ("1 mol N"_2)/(28.014 cancel("g N"_2))`

`~~ "0.892 mol",`

which is a smaller number and easier to work with.