Mass, Moles and Measuring

by Rosie Coates

“Rien ne se perd, rien ne se crée, tout se transforme.”

“Nothing is lost, nothing is created, everything is transformed.”

This is the law taught to French students routinely and paraphrased from Antoine Lavoisier’s “Traité Élémentaire de Chimie“. It sounds very philosophical doesn’t it? Even more so in French, but it stems from all that is fundamentally chemistry, (to my mind a least) burning stuff and measuring it. This blogpost explores the history, experiment and theory of the conservation of mass and moles.

Antoine Lavoisier

Chemistry: Cute and cuddly (Photo: Mike E Talbot, CC-BY-SA)

Chemistry: Cute and cuddly (Photo: Mike E Talbot, CC-BY-SA)

Antoine Lavoisier is often credited as the ‘father of chemistry’, I’m not convinced of that*, but his work on quantifying chemistry (and doing a lot of measuring along the way) was a major factor in the path that chemistry took in the century following his work.

*You can find out why he’s not my top dead chemist and who is, in this Pythagoras’ Trousers lecture.

Lavoisier Balance for weighing products of combustion (Image: Madame Lavoisier, 1789, Public Domain)

Lavoisier Balance for weighing products of combustion (Image: Madame Lavoisier, 1789, Public Domain)

Lavoisier was able to show, using some of the equipment shown in these diagrams (skillfully created by his wife), that metals increased in mass when they were burned.

By carefully weighing the whole apparatus in a sealed vessel he was able to establish that the total mass was conserved.  Even though the metal gained mass, this mass must have been lost from the air inside the vessel.

If you fancy trying out  some burning and weighing why not try this lovely, simple class demo from the RSC learn-chemistry pages.

This gave us the concept of conservation of mass. Why is that important?

Chemistry is all about control

We want to know what we can make and how much of it we can make. Knowing that mass is going to be conserved in all your reactions is the first step.  From there we can start to think about how much of each reactant we’re putting in and how much of each product we’ll get out.  To help us do this we use equations like this:


If you’re doing this reaction you might be trying to make potassium hydroxide (KOH), perhaps so that you can use it to make some liquid soap (which is what a huge amount of potassium hydroxide is used for). In which case, you need to know how much potassium (K) and water (H2O) you need to react together to get the amount of product you’re hoping for.

How many moles?

The equation tells us that for every two moles (parts) of potassium we react we will get 2 moles of potassium hydroxide and one mole of hydrogen gas.  Then we use the periodic table to tell us the Relative Molecular Mass (RMM) of the reactant and products.


I am going to put in 78g of potassium can you work out how much potassium hydroxide I will get out?

The mole triangle

We can use the handy mole triangle to help us. To get moles you divide mass by RMM.  To get mass you multiply moles by RMM (and if you want to find out the RMM you can divide mass by moles).


First we need to find out how many moles in 78g potassium?

78g ÷ 39g/mol = 2 mol = 2 moles  (mol is the standard unit for moles.)

Looking back at the formula we know we will get 2 moles of KOH from our 2 moles of Potassium. What mass will this be?

2 x 56g = 112g.

We would expect 112g of KOH to be produced in this reaction, as long as we had enough water for all the potassium to react.  In practice that would be a pretty massive reaction. The clip below shows what will happen when just a few grams of potassium reacts with water, imagine 78g!!

That sort of thing could get nasty, which is why, as I mentioned before, we chemists really do like to be in control of our reactions.  Getting to grips with mass and moles and measuring is the first step to doing so!

Curriculum Links


Key stage 3- Atoms, elements and compounds

conservation of mass, changes of state and chemical reactions


Key stage 3- The Sustainable Earth

Investigations into the patterns of behaviour of elements and compounds and their use to describe and predict their behaviour in chemical reactions

Key stage 4- Chemical and Material Behaviour

There are patterns in the chemical reactions between substances.


Materials- Properties and Uses of Substances

Through evaluation of experimental results, I can demonstrate my understanding of conservation of mass.

SCN 4-16b


Mole calculations


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