For example, when hydrogen (H2) reacts with iodine (I2) to produce hydrogen iodide (HI), we can write a balanced chemical equation for the reaction as: H2 + I2 –> 2HI.
Now, let’s call this chemical equation as equation A:
H2 + I2–> 2HI
To interpret equation A in terms of moles,
we will say that 1 mole of H2 molecules react with 1 mole of I2 molecules to give 2 moles of HI molecules.
As a result, we can write the following expression:
1 mol H2 = 1 mol I2 = 2mol HI
From the expression we can write mole-mole ratio between any two chemicals. That is, we can write the ratio between H2 and I2, as 1 mol H2/1 mol I2 or 1 mol I2/1 mol H2. Note that the slash bar means division
Similarly, we can write the ratio between HI and H2 as 2 mol HI/1 mol H2 or 1 mol H2/2 mol HI
It’s sometimes confusing to interpret the mole this way. To ease this confusion a little, we can relate a mole to a dozen. For example, we can say that: 1 dozen H2 molecules reacts with 1 dozen I2 molecules to give 2 dozen HI molecules.
Chemists usually count individual atoms or molecules by weighing a bunch of them. This means that it’s more useful to interpret the chemical equation in terms of moles. Once we do that, we can use the mole concept to work back to figure out the number of molecules in a substance.