Figure \(\PageIndex{1}\) shows that we need 2 hydrogen atoms and 1 oxygen atom to make one water molecule. If we want to make two water molecules, we will need 4 hydrogen atoms and 2 oxygen atoms. If we want to make five molecules of water, we need 10 hydrogen atoms and 5 oxygen atoms. The ratio of atoms we will need to make any number of water molecules is the same: 2 hydrogen atoms to 1 oxygen atom.
Figure \(\PageIndex{1}\) Water Molecules. The ratio of hydrogen atoms to oxygen atoms used to make water molecules is always 2:1, no matter how many water molecules are being made.
Using formulas to indicate how many atoms of each element we have in a substance, we can relate the number of moles of molecules to the number of moles of atoms. For example, in 1 mol of water (H2O) we can construct the relationships given in (Table \(\PageIndex{1}\)).
Table \(\PageIndex{1}\): Molecular Relationships for Water
A mole represents a very large number! The number 602,214,129,000,000,000,000,000 looks about twice as long as a trillion, which means it’s about a trillion trillion.
A trillion trillion kilograms is how much a planet weighs. If 1 mol of quarters were stacked in a column, it could stretch back and forth between Earth and the sun 6.8 billion times.
Table \(\PageIndex{2}\): Molecular and Mass Relationships for Ethanol
How many molecules are present in 16.02 mol of C4H10? How many C atoms are in 16.02 mol?
How many moles of each type of atom are in 2.58 mol of Na2SO4?
Answer a:
9.647 x 1024 C4H10 molecules and 3.859 x 1025 C atoms
Answer b:
5.16 mol Na atoms, 2.58 mol S atoms, and 10.3 mol O atoms
Summary
In any given formula, the ratio of the number of moles of molecules (or formula units) to the number of moles of atoms can be used as a conversion factor.