What are moles and how do you use them?

A mole is an amount of an element or compound which contains as many atoms or molecules as is present in 12g of carbon 12 (6.02x10²³ Avogadro’s number).

We can find out how much this is by looking at the relative atomic mass (A_r) of an element (which can be found on a periodic table).

For example, the A_r of calcium is 40, so 1 mole of calcium would weigh 40g.

For compounds, such as calcium carbonate, you just need to add up the A_rs of each compound, giving you the relative formula mass (M_r). The formula for calcium carbonate is:

CaCO₃

So we add up the A_rs:

40+12+16x3=100

Calcium carbonates M_r is 100 therefore 1 mole of it would have the mass 100g.

You can use moles to work out the reacting masses of chemical reactions. For example:

CaCO₃  -> CaO + CO₂

 If we wanted to know how much calcium oxide would be made by thermally decomposing 50g of calcium carbonate. first we would work out how many moles of calcium were reacting. To find this out we use the equation:

   M_r x Moles = Mass

It helps to think of it as a triangle:

       Mass 

    M_r x Moles

When you want to know one of these three things you just cover it up and what is left makes up the equation. So for example we want to know Moles, we cover it up giving:

Mass/ M_r

Therefore:

50/100=1/2 Moles of CaCO₃

The equation states we produce 1 mole of calcium oxide for every 1 of calcium carbonate therefore we produce half a mole of calcium oxide. Now to find the mass of calcium oxide we use our equation again, this time mass is covered up so:

M_r x Moles

The M_r of calcium oxide is 56 therefore:

56 x 1/2= 28g of calcium oxide are produced.