For example, when hydrogen (H_{2}) reacts with iodine (I_{2}) to produce hydrogen iodide (HI), we can write a balanced chemical equation for the reaction as: **H _{2} + I_{2}** –>

**2HI.**

Now, let’s call this chemical equation as equation A:

**H _{2} + I_{2}**–>

**2HI**

### To interpret equation A in terms of moles,

we will say that 1 mole of H_{2} molecules react with 1 mole of I_{2} molecules to give 2 moles of HI molecules.

As a result, we can write the following expression:

**1 mol H _{2} = 1 mol I_{2} = 2mol HI**

From the expression we can write mole-mole ratio between any two chemicals. That is, we can write the ratio between** H _{2} and I_{2}, as 1 mol H_{2}/1 mol I_{2} or 1 mol I_{2}/1 mol H_{2}.** Note that the slash bar means division

Similarly, we can write the ratio between HI and H_{2} as **2 mol HI/1 mol H _{2} or 1 mol H_{2}/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 H _{2} molecules reacts with 1 dozen I_{2} 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.