There’s a mole in our midsts!

A close up of a mole

I'm a mole of the furry variety!

No, not the poor-sighted, furry kind or the undercover, secret agent sort. I’m talking Chemistry and Physics here and the wonderful SI unit that describes the amount of stuff!

So, what is “a mole”?

The mole is the amount of a substance of a system which contains as many elementary entities as there are atoms in 0.012 kilogram of carbon-12; its symbol is “mol.” When the mole is used, the elementary entities must be specified and may be atoms, molecules, ions, electrons, other particles, or specified groups of such particles.

SI – Système International d’Unités
(International System of Units)

The unit of a mole provides a convenient and consistent way to convert  between atoms, molecules and grams. Think of it along the lines of how we use the word ‘dozen’ when we refer to 12 eggs, or a ‘byte’ when we talk about 8 bits in computing and you can start to see how a ‘mole’ can be a useful term to have in your arsenal!

You can see the definition of the mole above doesn’t actually mention something else that you might remember from your school chemistry days when it comes to moles – Avogadro’s Number – one of the fundamental constants of chemistry.

The Italian chemist, Amedeo Avogadro hypothosised in 1811 that “equal volumes of gases at the same temperature and pressure contain the same number of molecules regardless of their chemical nature and physical properties”.

Big Avo’s ponderings can be seen reflected in the ideal gas law which generally expresses the behaviour of many gases under different conditions quite well:

PV = nRT


  • P is the pressure
  • V is the volume
  • n is the amount of substance of the gas (in moles)
  • R is the gas constant (8.314 J·K−1mol-1)
  • T is the absolute temperature

Avogardo’s number is usually expressed as 6.022 x 1023 and it’s used extensively in stoichiometry – the branch of chemistry that deals with the relative quantities of reactants and products in chemical reactions. It refers to the number of entities (e.g. atoms, molecules, particles) in 1 mole.

1 mol = 6.022 x 1023

So, 1 mol of pure 12C has a mass of exactly 12 g

An easy way to work out the amount of pure substance whose molar mass is known, is to divide its mass by its molar mass:

Amount of substance = mol / g

In the chemical equation,

2 H2 + O2 –> 2 H2O

You would say that 2 moles of di-hydrogen and 1 mole of di-oxygen react to form 2 moles of water.


Density is mass per unit volume of a substance. The symbol most often used for density is ρ (the Greek letter rho).


ρ = Kg / L


A solution is usually a solid substance dissolved in a liquid, such as salt dissolved in water. It is important to know the concentration of a solution.

Concentration can be thought of as the ‘strength’ of a solution and is expressed as ‘molarity’.

Molarity is defined as the number of moles of a solute (mol) divided by the volume of the solution (L  or litres).

So, Molarity = moles of solute / litres of solution = mol/L

To work out, for example, the molarity of a solution formed by dissolving 5.00 g of Sodium Hydroxide (NaOH) in 5000 mL of water (H20) you could do the following calculation:

5.00 g NaOH / 1 x 1 mole /  (22.9 + 16.00 + 1.008) g = 0.125 mol

If, Molarity = moles of solute / litres of solution

Then, Molarity = 0.125 mol / 5.00 L = 0.025 mol/L




About Derek Shirlaw

I'm passionate about science communication, social media, and my home country, Scotland. In particular, I have a real interest in astronomy, digital marketing, and the great outdoors.
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