**Pretty obvious… Not So Obvious…**

This image represents a **Heliox 50, a mixture based on an equal number of elementary particles (atoms or molecules) of Oxygen and Helium.**

So, when we talk about gas percentages, we mean the relationship between **the amount of different gases expressed in terms of the number of elementary particles, atoms or molecules.**

Before going into the heart of the matter, it’s important to clarify a topic apparently obvious.

Let’s imagine preparing a Heliox 50 starting from an empty cylinder.

**What does Heliox 50% mean ?**

It seems provocative but instead it’s important to stress this point: half Helium and half Oxygen.

Obvious, right ? Only apparently, half and half in what terms?

For example, 100 bar Oxygen and 100 bar Helium are half and half, but this is not a Heliox 50.

If you add 100 bar Oxygen and then Helium to 200 bar in sequence at 20 °C to an empty cylinder, you get a Heliox 57.1

Reversing the order of the gases, Helium first, then Oxygen, you get instead a Heliox 47.6

And even 1,000 grams of Helium and 1,000 grams of Oxygen are half and half, but even this is not a Heliox 50.

The misunderstanding here is even bigger: the relevant difference between the atomic weights of Helium and Oxygen causes a Heliox 11.1 !

The unit of measure used by chemists to measure the amount of gas is the **mole**, which identifies precisely an exact number of particles.

The mole is widely used in chemistry and has a value of 6.023 * 10 ^ 23 elementary entities of the substance, atoms or molecules in our case. This number expressed in scientific notation is an amount of **six hundred thousand billions of billions** of entities.

As far as blending by partial pressure is concerned, the procedure does not depend on the capacity of the cylinder.

Therefore, we can then talk in terms of **concentration,** defined as **the number of moles per litre (mol/L) of volume**, in order to make the matter under discussion independent from the volume of the cylinder.

In other words, we are talking about how many elementary entities of a certain gas there are per liter of internal volume of the cylinder.

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