The Mole

Once we know the mass of a sample we can use the mass of the atoms or molecules in the sample to determine how many atoms or molecules are in the sample.

How many ¹²C atoms are in a 1.00 kg block of pure ¹²C isotope?

Obviously atoms and molecules are not a convenient unit of measure when we're working with macroscopic (i.e., human size) objects. For this reason chemist define a new unit of measure called the mole.

1 mole is defined as the number of carbon atoms in exactly 12.000000 grams of pure ¹²C.

From our definition of the atomic mass unit we get

The number 6.022140857 x 10²³ is called Avogadro's number.

How many moles are in a 1.00 kg block of pure ¹²C isotope?

Of course we know a 1.00 kg block of naturally occuring carbon will contain a mixture of ¹²C, ¹³C, and even some ¹⁴C isotopes. So the number of moles of carbon atoms in a 1.00 kg block of naturally occuring carbon is

We can also calculate the mass of a mole of molecules.

In other words the molecular weight of CH₄ is 16.043g.

Molecular weight = the mass in grams of 1 mole of a molecule.

For ionic compounds, which do not exist as individual molecules we use the term Formula weight

Formula weight = The mass in grams of 1 mole of the chemical formula.

Let's look at another example.

How many hydrogen atoms are there in 2.50 g of NH₃?

Now, let's try some sample quiz questions on: